Engineer Physics of Flight

8/14/2019 Engineer Physics of Flight

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The Physics Behind Flight

Chris RogersBen Erwin

Barbara Bratzel

1996 by C. Rogers, B. Erwin, and B. Bratzel


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ForceA Desire to move

Sum of all Forces = mass x acceleration

General Concept:A force is often better known to kids as a push or a pull (contact force). When John is pushing the

wagon up the hill, he is exerting a force on the wagon and hence the wagon moves. The moresubtle aspect of forces, however, comes in the idea of counter forces. Kim, standing on top of thehill, is exerting a force on the ground. That is, she has a certain weight (the force exerted on anobject by a gravitational field). Fortunately for her, however, the ground is exerting exactly thesame force upward, so the two forces balance out each other. The sum of all the forces (the netforce) exerted on Kim is zero and she does not move.

Now, it is a little-known fact that Kim is actually standing on a bunch of branches covering a hole.John, who dug the hole, tosses Kim a heavy ball. Now the weight of Kim and the ball is greaterthan the counter-force that can be exerted by the branches, the sum of all the forces are no longerzero - and Kim goes flying into the hole. She accelerates downward.

General Facts:

You can exert a force by pushing or pulling somethingIf all the forces on an object are equal and opposite, the object will not accelerate.If the forces are not equal and opposite, the object will accelerate in the direction of the net force.

Example: the carPushing the gas pedal translates to the tires pushing the car forward against the road.If the tires are pushing with a force that exactly counteracts all other forces (wind resistance, tirefriction*, etc.) the car will neither accelerate nor decelerate.(Sum of forces = 0, therefore acceleration = 0)If the wind resistance, tire friction, etc. exceed the tire push, the car will slow down.(Sum of forces < 0, therefore acceleration < 0)If the tire push is greater than the wind resistance etc., the car will accelerate.(Sum of forces > 0, therefore acceleration > 0)

* friction is the force between two things when they make contact

Friction

wind

tire push


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TorqueA desire to rotate

Sum of all Torques = rotational inertia x angular accelerationTorque = Force x Moment Arm

General Concept:

A torque is often better known to kids as a rotation. When Jill tightens the bolt with a wrench, sheis exerting a torque on the bolt. Like the force, if all torques are equal, she will be unable to tightenthe bolt. If the torque she exerts is greater than the counter torque of the friction in the bolt, thebolt will rotate (tighten).

Torque and force are directly linked. The more Jill pushes (applied force) at the edge of thewrench, the more torque she applies and the tighter the bolt becomes. However, it is not just forcethat makes a difference. The further out from the bolt she holds the wrench, the more torque sheapplies, and the tighter the bolt. Therefore, the torque must be related to both the push (force) anddistance from the center of rotation that the force is applied. This distance is called the momentarm. A similar example is a door. The further away from the hinges you push on a door, themore torque you are applying, and the easier it is to open.

General Facts:You can exert a torque by pushing or pulling something a given distance from the center ofrotation. You must push perpendicular to the moment arm. If all the torques on an object are equaland opposite, the object will not rotate faster or more slowly (no angular acceleration). If thetorques are not equal and opposite, the object will rotate faster in the direction of the net torque.

Example: the pedal on the bikePushing the bike pedal translates into a torque which rotates the tires. If you are exerting a torquethat exactly counteracts all other torques (frictional torques, etc.) the tire (pedal) will neither

accelerate nor decelerate.(Torque sum=0, therefore angular acceleration=0)if the frictional torques etc. exceed the torque you exert, thetire (pedal) will slow down.(Torque sum0)

force

arm


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EnergyThe capacity to do workSum of all energy = 0

General Concept:The total amount of energy must stay constant. That means that if you use energy to do

something, it is at the cost of something else. For instance, Dave the Olympic diver uses energythat he got from food to climb to the top of the high dive. He has converted that food energy(calories) into potential energy. He jumps off the board (after exerting as big a torque as possibleon the board tip) and converts this potential energy into kinetic energy (energy of motion) as heaccelerates down to the pool. Note that he has unequal forces exerted on him - his weight pullinghim down and air resistance trying to keep him up - and since they are unequal (his weight is alarger force), he accelerates down until he hits the water. When he hits the water, he dissipates theenergy into the water, climbs out, and does the whole thing over again.

There are numerous forms that energy can take. The most common are:Potential energy (energy associated with height) = weight x heightKinetic energy (energy associated with movement) = 1/2 x mass x velocity x velocityPhoton energy (energy associated with light)

Electricity (energy associated with the motion of electrons)Heat (energy associated with the motion of molecules, measured by temperature)Sound (energy associated with acoustics)

General Facts:Energy is conserved; that is, you can only transfer energy from one form to another.Unlike force and torque, energy has no direction associated with it.

Example: dropping a bookThe book on the shelf has some potential energy associated with its height.As the book falls off the shelf it looses height (potential energy) and increases in speed (kineticenergy).(Decrease in potential energy = increase in kinetic energy)

When it hits the ground it stops (loses kinetic energy) but makes a noise (acoustic energy).(Loss of kinetic energy = increase in acoustic energy (and some heat))

Even though energy is conserved, you can not always control the direction or form that it takes.For instance, no amount of yelling at the book will lift it back up to its original height.


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PressureThe exertion of force upon a surface

Pressure = force / area of surface

General Concept:Pressure is often a difficult concept for the students to grasp. It is the total force exerted per unit

area.* Fred the fisherman feels this pressure as he walks through the stream searching for thatever-elusive fish that got away. The boot feels tighter under water - because the water isapplying a greater pressure at the bottom of the stream than at the top.

Although the pressure at any point is equal in all directions, the force resulting from a pressurealways acts perpendicular to the surface. When you put your hand out the car window, airpressure in front of your hand pushes the hand back with a greater force than the force due to airpressure behind the hand, therefore accelerating the hand backward.

When you stand on the balls of your feet, they will start to hurt quickly. Although the force is thesame (your weight), the area that the force is acting on has gotten smaller, and you feel a greaterpressure.

There is pressure on you all of the time. The weight of the atmosphere, divided by the surface areaof the earth, is called the atmospheric pressure. When you go on top of a mountain the pressureis less because there is less atmosphere above you to push down on you.

General Facts:Pressure can result from molecules of air (or water) hitting you - there is no pressure in outer spacewhere there are no molecules.The force resulting from a pressure is perpendicular to the surface.A fluid always wants to move from a high pressure to a low pressure.

Example: the nailjust like the balls of the feet example: same force, different pressurespush both the head and the tip down onto your finger

since the tip area is much smaller than the head area, the pressure is much greater when you pressthe tip onto your finger - and therefore hurts morethis is why you want to sharpen your knives. The sharper the knife, the smaller the area, thegreater the pressure - and the easier it is to cut.

*unit area is a 1x1 area in whatever units you are working in. i.e. inches would be 1x1 square.example: atmospheric pressure is 14.7 psi: 14.7 pounds of force on every square inch.


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VelocityMotion

average velocity = distance traveled / elapsed time

General Concept:A velocity is often better known to kids as a speed. The faster Becky runs, the greater her

velocity.

General Facts:velocity has a direction associated with it

Example: toy carmeasure out a distance on the floortime how long it takes for a toy car to go the length of the floorfind the average velocity of the car (distance covered/time taken)


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Bernoullis TheoremHow pressure and velocity interact

static pressure + dynamic pressure = total pressure = constantstatic pressure + 1/2 x density x velocity^2 = total pressure = constant

General Concept:

The Bernoulli effect is simply a result of the conservation of energy. Energy can be stored as anincrease in pressure or as a change in velocity. Although one can take into account changes inenergy due to height changes (potential energy), or energy losses due to friction, etc., we willassume that they are small for now.

General Facts: If you slow a flow down, the pressure will go up. You can speed up a flow by dropping the pressure.

In the real case, friction plays a large role - a lot of times you must have a large pressure drop(decrease in pressure) just to overcome friction. This is the case in your house. Most water pipeshave small diameters (large friction), hence the need for water pressure - it is the energy fromthat pressure drop that goes to friction.

Example: the showerheadA showerhead (if you have a fancy one) has a number of different operation modes. If you go forthe massage mode, you are moving a little water fast. For the lite shower, you are moving alot of water slowly. It takes the same amount of energy to move a little water fast as it does tomove a lot of water slowly. this is the amount of energy you have due to your water pressure.

high velocitylow pressure

high pressure

low velocity


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Lift and DragTwo of the four forces on an airplane

General Concept:Just like in the first page of this manual, an airplane flies by balancing forces. If the trust of theairplanes engines is greater than the force of the wind (drag force), the plane will go forward.

Likewise, if the lifting force of the wings is greater than the weight of the plane (the force ofgravity), the plane will rise in the air. Therefore, to make the airplane work, one must havesufficiently powerful engines to push the plane through the air, and have designed wings to lift theplane in the air. Once in the air, airplanes maneuver by moving control surfaces which providetorques about the center of mass of the airplane. These torques will rotate the aircraft in theappropriate direction.

Forces on an airplane

Control surfaces on an airplane


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LiftUpward Force (good!)

General ConceptThe force of the wind on the plane can be divided into two components: a component pushing theplane up and a component pushing the plane back. The upward force, the lift force, is what keeps

the plane in the air. In fact, the pilot can change the lift force: getting a lot of lift at takeoff (need toaccelerate the plane upward) and less lift during cruise (just need to overcome the weight of theplane.

Before delving into what causes lift, it is a good idea to define some parts of the wing:Definitions: leading edge: the part of the wing that sees the air first (faces the direction of

motion)trailing edge: the rear edge of a wingchord line: the line joining the leading edge and trailing edgeangle of attack: the angle between the chord line and the incoming wind.

leading edge

trailing edge

chord line

Angle of attack

Flow

Wing definitions

A streamline of a fluid flow is like a snapshot of the flow at any given time. The velocity of theflow is always tangent to the streamline.

Streamlines around an airfoil

Streamlines vs. Pressure:Streamlines actually tell one a lot about the flow. They show you how the flow is moving aroundan object. They also show you what the pressure looks like everywhere around the object. Itturns out that if there is curvature in the streamlines, then there must be a change in pressure. Infact, it is that change in pressure that is keeping the streamlines curved. If there where to pressuredifference, then the streamlines would be straight. Further, the lowest pressure is always at thecenter of curvature. Therefore, sharp curves correspond to low pressures. So if I can design ashape that has sharp curves on the top and smooth on the bottom, then I will get a pressure


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difference across that surface. This is how the airfoil works.

High Pressure (little curvature)

Low Pressure (sharp curvature)

Net Lift Force

Pressure difference causes the net lift force

As one increase the angle of attack of the airfoil, this curvature over the top becomes greater -causing a greater pressure difference and therefore a greater lift force. Eventually, however, if thecurvature becomes too great, the flow separates off the wing and you have stall. With stall comesa drastic change in the curvature and therefore the pressure difference and therefore the lift. Stallusually causes the pilot to loose some control until he decreases the angle of attack and regainssubstantial lift (all planes have stall sirens that go off when the wings stall).

Net Lift Force

Stall: too high an angle of attack reduces the lift (and curvature)

Another way to explain lift is through circulation:

Initially, air flowing under the wing tries to curl around thetrailing edge and around to the top surface. Such a quick turnrequires an infinitely high velocity, which is not desirable bynature. Viscosity (friction) causes the flow to begin to leave thetrailing edge smoothly. This condition at the trailing edge

results in the curl, or the circulation, to be shed. Becauseenergy is conserved, another circulation in the oppositedirection becomes attached to the wing. The same effect can beobserved when moving a spoon through a teacup. Note the tworegions of circulation - in opposite directions.

SpoonCoffee Cup


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Circulation of opposite direction at the start of the airflowover an airfoil

The trailing edge circulation gets left behind as the wing moves forward, but the clockwisecirculation remains attached to the wing. This circulation causes the flow over the top of the wingto be faster than the flow over the bottom of the wing. Because of Bernoullis theorem, the fasterflow causes the pressure on the top surface of the wing to be low. This is what causes lift. Thetwo circulation regions can be seen by running an airfoil through a pan of water.

Lift can occur on objects other than wings. Lift is responsible for golf balls slicing into the woodsand top spin lobs landing in the tennis court. To illustrate the top-spin lob, imagine the ball in thepicture below traveling to the right while also spinning clockwise. The balls spinning motion isadding circulation to an otherwise uniform flow. It is the addition of these two types of flow -circulation and uniform flow - that creates lift. You need them both. The spinning of the bottomof the ball tends to add velocity to the flow around the ball. Because of the Bernoulli effect, thisdecreases the pressure there, and the ball tends to go downward (negative lift). This is why top-spins tend to land in the court.

Net Lift Force


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DragForce against direction of motion (bad!)

General ConceptDrag is the force that slows the airplane down. It is defined as all forces opposite the direction ofmotion of the plane. These forces are usually one of two types: pressure drag and skin friction

drag.

Pressure Drag occurs when the air flow separates from the surface. If you stick your palm outand swing it through the air (thumb towards ceiling and pinky closer to floor - that is give it a slightangle of attack), your hand experiences pressure drag, much like the picture of the wire below.

Skin Friction Drag is the drag due to the friction between the air and the surface movingthrough the air. If you slice your hand through the air, palm parallel to the floor, your hand isexperiencing mainly skin friction drag (the surface of an airplane wing is also called a skin). Thefriction is experiences because your hand tends to try to pull the air alongwith it.

A wing can have the same drag as a wire that is much smaller than the wing. The difference is thatthe wing has mostly skin friction drag, and the wire has mostly pressure drag. Can you see why

airplanes no longer have lots of wires like the old bi-planes used to?

Pressure drag on a wire versus a wing

Another kind of pressure drag occurs at the tips of the wings. This is called induced dragbecause it is caused by, or induced by the lift on the wings. Air always wants to move fromhigh pressure to low pressure. Because the air above the wing is at a low pressure, the air wantsto curl up around the top of the wing. As the wings move through the air, this curling actioncauses big spirals (vortices) of air. these vortices have a lot of kinetic energy. The engineshave to produce extra power to overcome the energy that these vortices create. Because thesevortices make the engines work harder, impede the motion of the plane, and are caused by the lift,they are often referred to as drag due to lift.

low pressure

high pressure high pressure

low pressure

Wing tip vortex formation


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wingtipvortices

Two wingtip vortices (induced drag)

These wingtip vortices are the main reason that planes must wait a while between takeoffs. It canbe dangerous flying in the vortices of another plane. These same vortices are the reason that birdsfly in V-formation. In this case, the birds take advantage of the upwind side of the vortexshedding off the bird in front of them. This updraft actually lifts the bird up, making the flight a

little easier.


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B low ing C ans

What happens when you blow bet ween two cans?

Mat erials:

approximat ely 2 5 drinking st raws

t wo empt y soda cans

Inst ruct ions:

1. Place t he st raws on a f lat t able t op, parallel t o each ot her and

about 1 cent imet er apart .

2. Place t he cans upright on t he st raws, about f ive cent imet ers

apart.

3. What do you t hink will happen if you blow air bet ween t he cans

t hrough a st raw? Try i t .

What happened?

Why?

Reference:

Jacobs, S. (1 99 4) . Whelmers, Volume 1: Science Demonst rat ions t hat

Spark your Imaginat ion. Wichit a, Kansas: Jakes At t ic Product ions.

( Dist ribut ed by Sargent -Welch Scient ific Company, 1 -8 00 -SARGENT) .


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Bl o w i n g o n Pa p e r : T w o A c t i v i t i e s

Blowing on paper can have surprising result s.

B l o w i n g P a p e r o f f B o o k s

Mat erials:

paper

t wo t hick books

met r ic ru ler

Inst ruct ions:

1. Lay t wo t hick books about 10 cm apart .

2. Place a sheet of paper on t he books so t hat it bridges t he gapbetween t hem.

3. Try t o blow the paper off t he books by blowing underneat h it .

What happens? Why?

Bl o w i n g S h ee t s o f Pa p e r A p a r t

Mat erials:

t wo sheet s of paper

` m et r ic ruler

Inst ruct ions:

1. Hold two sheet s of paper vert ically about 5 cm apart .

2. Blow t he sheet s apart by blowing hard bet ween t hem.

What happens? Why?

References:

Walpole, B. (19 88 ). 175 science experiments t o amuse and amaze your

f r iend s. New York: Random House.


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The Hover ing Card

Blow downwards on a card wit h surprising result s.

Mat erials:

index card

scissors

met r ic ru ler

penci l

thumbtack

spool

s t r aw

Inst ruct ions:

1. Cut a square 5 cm on each side out of t hin cardboard. (An index

card is about t he right t hickness.)

2. Draw diagonal lines connect ing t he opposit e corners. The lines

should cross at the center of the card.

3. Caref ully push a t humbt ack t hrough t he point where t he lines

int ersect . Hold t he card so that t he flat part of t he t humbt ack

is underneath and it s point is on t op.

4. Place a spool over t he point of t he t ack. Insert a short piece of

a st raw int o t he hole on t he top of t he spool.

5. Blow hard t hrough t he straw, t hen let go of t he card.

What happens? Why?

References:

Dohert y, P. and Rathjen, D. (1991) . The Explorat or ium Science Snackbook.

San Francisco: Explorat orium Teacher Inst it ut e.




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B lo w i n g o n Pi n g - Po n g B al l s : T w o A c t i v i t i e s

P ing -Pong Ba l l i n a Funne l

Can you blow a ping-pong ball out of a funnel?

Mat erials:

ping-pong b all

funnel

Inst ruct ions:

1. Place a ping-pong ball in a clean funnel.

2. Hold t he f unnel upright and put t he narrow end in yourmouth.

3. Blow t he bal l out of t he funnel.

What happens? Why?

Reference:

Churchill, E. R. (1991) . Amazing Science Experim ent s wit h Every day

Mat er ials . New York: St erling Publishing Co.

Suspended P ing -Pong Ba l l s

Can you blow t wo ping-pong balls apart ?

Mat erials:

t wo ping-pong balls, or ot her small ballss t r ing

tape

met r ic ru ler

Inst ruct ions:


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1. Tape a piece of st ring t o each ping-pong ball.

2. Hang t he balls at t he same level about 5 cm apart .

3. Blow hard bet ween t he t wo balls.

What happens? Why?

Reference:




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Un r o l l i n g T o i l e t Pa p e r

Theres more t han one way t o get t oilet paper off t he roll.

Mat erials:

ro l l o f t o i le t paper

leaf blower

dowel rod, about t wo feet long

Inst ruct ions:

1. Thread t he dowel rod t hrough t he t oilet paper roll. Unroll

about t wo feet of t oilet paper, lett ing i t hang down.

2. Have one person hold t he dowel horizont ally, wit h t he hangingpaper f acing away f rom t he leaf blower.

3. A second person should hold t he leaf b lower. Turn on t he leaf

blower, direct ing t he st ream of air just over t he t op surface of

t he roll. What happens?

When t he leaf blower is t urned on, what happens t o t he pressure above t he

rol l o f t o i le t paper?

Why does t he paper unroll?

Reference:





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W a t e r Sp r a y e r

How do perfume and plant sprayers work?

Mat erials:

t wo st raws

drinking glass

Inst ruct ions:

1. Hold a straw upright in a glass of wat er so t hat t he top of t he

straw project s over t he top of t he glass.

2. Place a second st raw perpendicular t o t he first so t hat t he end

of t he second st raw is almost t ouching t he opening of t he

f i rs t , but is not b locking it .

3. Blow hard t hrough t he second st raw.

What happens? Why?

References:

Walker, J. (1 97 7) . The Flying Circle of Physics wit h Answer s. New York:

John Wiley & Sons.




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Paper shapes

Mat erials:

Index cards or t hin cardboard

ruler

scissors

scot ch t ape

blow dryer

Inst ruct ions:

1. Cut t wo (or more) index cards int o strips about 2 cm wide.

2. Bend one st rip int o a circle and t ape t he two ends t ogether.

3. Form t he remaining st rips int o ot her shapes: square, t eardrop,etc.

4. Place t he shapes in a row on a t able.

5. Place t he blow dryer so i t s air st ream wil l f low along t he

surface of t he t able. Turn it on low and t ry blowing t he shapes

off t he t able.

The shapes t hat shift t he most easily have t he most drag, or air

resist ance. Those t hat m ove least are t he most st reamlined.

Which of y our shapes had t he least drag? Can you design an even more

st reamlined shape?

Reference:

Herbert , D. and Ruchlis, H. (1968 , revised 1983 ) . Mr. Wizards 400

Exper iment s in Science. Nort h Bergen, N.J.: Book Lab.


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B o u n c i n g B al l s

How high can a ping-pong ball bounce?

Mat erials:

ping-pong b all

gol f bal l

hard f loor

Inst ruct ions:

1. Drop t he ping-pong ball f rom about chest height . How high does

it bounce?

2. Can you t hink of ot her ways t o make t he ping-pong ball bouncehigher? To do so, you must give t he ball addit ional energy.

(For example, dropping it from a greater height would increase

it s potent ial energy.) How else could you add more energy?

3. Drop t he golf ball f rom t he same height . How high does it

bounce?

4. Now hold t he two balls in one hand at chest height, wit h t he

ping-pong ball direct ly above t he golf ball. Drop t hem. How

high does each ball bounce?

When t he balls are bounced t oget her, t he ping-pong ball must have more

energy (since it bounces higher). Where does t he ext ra energy come f rom?

What about t he golf ball? Does it have more or less energy when bouncedwit h t he ping-pong bal l?

What do you t hink would happen if you bounced t he two balls wit h t he golf

ball on top?


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What would happen if you bounced t wo ping-pong balls t oget her?

Reference:





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L inked Pendu lum s

St art a pendulum swinging wit hout t ouching it.

Mat erials:

t wo chairs (and perhaps books t o weigh t hem down)

s t r ing

modeling clay

Inst ruct ions:

1. Make t wo balls of modeling clay, each about t he size of a ping-

pong ball. At t ach each ball t o a 40 -cm-long st ring.

2. Tie a t hird piece of st r ing t o t he backs of t wo chairs f acingaway f rom each ot her about one met er apart .

3. Tie t he t wo modeling-clay pendulums t o t he st ring bet ween t he

chairs, spacing t hem evenly. Make sure t hat t he pendulums are

hanging at t he same level.

4. Hold one pendulum st i l l while you st art t he ot her one swinging.

Now let go of t he first pendulum.

What happens? Why?

At each st age, which pendulum has more kinet ic energy?

Is energy conserved in t his system ( i.e., does t he t ot al amount of energy of

all t ypes st ay t he same) ?

References:






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Pa i n t - c a n Pe n d u l u m

Mat erials:

one-gal lon paint can wit h l id

sand

strong rope

large eye bolt

Inst ruct ions:

1. Fil l t he paint can wit h sand and close t he lid.

2. At t ach the eye bolt t o t he ceiling. Use t he rope t o suspend t he

paint can f rom t he eye bolt . The can should hang at about knee

level.

3. Pull t he can t o one side so t hat it just r eaches your nose. Let

go, being careful not t o push t he can or t o move.

Do you get whacked in the head? Why not (hopefully!)?

At what point is t he pot ent ial energy of t he can the great est ? The kinet ic

energy?

Reference:




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Ve lc ro C ans

Mat erials:

t wo ident ical cookie t ins wi t h l ids

adhesive-backed hook-and-loop fast eners

t en large met al washers

a board t o serve as a ramp

Inst ruct ions:

1. Using t he velcro, at t ach f ive washers t o t he inside of each can.

In one can, st ack t he washers in t he middle of t he can. In t he

other, arrange them evenly around the perimeter of the bottom

of t he can.

2. Put t he lid on each can. Place t he cans on t heir sides at t he t op

of a ramp.

3. Release t he cans at t he same t ime, being careful not t o give

eit her one a push.

Which can reaches t he bot t om f irst ? Why do you t hink t his happened?

Since t he t wo cans st arted wit h t he same amount of pot ential energy, why

didnt t hey f inish in a t ie?

Try ot her arrangement s of washers. Which ones are f ast er?


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Reference:




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B al l i n a n A i r St r e a m

Suspend a ping-pong ball in a st ream of air.

Mat erials:

blow dryer

ping-pong b all

Inst ruct ions:

Hold a blow dryer so t hat t he air st ream is direct ed st raight up.

Place a ping-pong ball in t he air st ream.

Once t he ball is suspended, t ry pull ing it a li t t le t o one side and let t ing go.

What happens? Why?

References:



Walker, J. (1 97 7) . The Flying Circle of Physics wit h Answer s. New York:

John Wiley & Sons.


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Bo t t l e Fo r c e m et e r

Use a soda bot t le t o measure balanced and unbalanced f orces.

Mat erials:

t wo- l it e r p last ic soda bot t le w i t h lid

masking t apepermanent marker

meter s t ick

food coloring

Instructions:

1. Fill t he bot t le full of wat er and add a couple of drops of f ood

coloring. Adjust t he wat er level so t hat when t he bot t le is

capped and laid on it s side, it cont ains one air bubble, aboutt he size of a nickel.

2. Put t he bot t le on a level surface. Tap on t he bot t le unt i l t he

air bubble comes t o rest .

3. At t ach a st rip of masking t ape along t he lengt h of t he bot t le.

Make a mark on t he t ape above t he air bubble and label t hat

mark 0.

4. From t he zero mark, draw marks on t he t ape in bot h direct ions,one cent imet er apart . Number t hem 1 , 2 , 3 , ... in each direct ion.

5. When your bubble is at zero, t he f orces on your bot t le are

balanced. If t he forces are not balanced, t he bubble will show

you the size and direction of the net force.


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Try moving your bott le t o t he right or left at diff erent speeds. What

happens?

Try moving your bot t le at a const ant speed. Where is t he bubble locat ed?

What happens if you speed up or slow down?

What happens if you spin t he bot t le?

References:

American Chemical Societ y/ American Inst it ut e of Physics (May 1 9 92 ) .

WonderScience volume 6, number 5 : Forces.


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D o - i t - y o u r s e l f S p r i n g Sc a l e

Make a spring scale and use it t o measure t he force of gravit y.

Mat erials:

shoe box wit h l id

t wo paper cl ips

rubber band

st r ing

paper cup

marker

whit e paper

tape

met r ic ru ler

nai l

pennies

Inst ruct ions:

1. Tape whit e paper t o t he lid of t he shoe box. Draw a st raight

line lengt hwise down t he cent er of t he lid. Punch a hole on t he

line near one end of t he lid.

2. Cut t he rubber band and t hread one end t hrough t he hole in the

lid. Tie one of t he paper clips on t he end of t he rubber band


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inside t he lid t o keep t he rubber band fr om sliding t hrough t he

hole.

3. Tape t he lid t o t he box. Tie t he second paper clip t o t he ot her

end of the rubber band.

4. Punch t hree holes in t he cup near t he rim, evenly spacing t he

holes around t he rim. Thread a st ring t hrough each hole. Tie a

knot in each string so that it does not pull through the hole.

Tie t he other ends of t he three st rings t oget her a few

centimeters above the cup so that the cup hangs evenly.

5. Tie a f ourt h piece of st ring t o t he knot holding t he ot her t hree.

At t ach t he ot her end of t he string t o t he paper clip on the

rubber band.

6. Hold t he box upright , t i lt ed forward sl ight ly so t he cup swings

f reely. Mark t he cent er line on t he box level wit h t he bot t om

of t he paper clip. Mark it 0.

7. Draw marks below t he zero mark at one cm int ervals. Number

t hem 1 , 2, 3 , and so on.

8. You can use your spring scale t o measure t o f orce of gravit y

upon any object you put in the cup.

Try put t ing a f ew pennies in your cup. If y ou double t he number of

pennies, does t he force double?

What happens t o t he force reading as you drop t he box? Why?

References:

American Chemical Societ y/ American Inst it ut e of Physics (May 1 9 9 2) .

WonderScience volume 6, number 5: Forces.




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D ropp ing Th ings

Youv e probably heard of Galileo dropping object s from t he Leaning

Tower of Pisa t o show t hat all object s fall at t he same rat e, regardless of

t heir weight . You probably also know from experience t hat object s dont

f all at t he same rat e. The discrepancy is due t o air resist ance. In a

vacuum, al l object s do fal l at t he same rat e. The f ol lowing act ivi t ies wi l l

help you examine t he opposing f orces of gravit y and air resist ance.

Mat erials:

book

sheet s of paper

sugar cube

dieball bearing

marble

ot her pairs of object s of t he same size and shape but diff erent

weights

Inst ruct ions:

1. Choose a pair of object s of t he same size and shape but

diff erent weight s. Hold t hem at t he same height and drop t hem

at t he same t ime. Do t hey hit t he ground at t he same time?

2. Cut a sheet of paper so t hat it has t he same lengt h and widt h

as t he book. Hold t he t wo object s horizontally at t he same

height and drop them t oget her. The book should fall f ast er.

Now place t he sheet of paper on t op of t he book and drop t hem

again. Does t he paper f all as f ast as t he book?

3. Take two identical sheet s of paper. Crumple one into a t ight

ball. Leave t he ot her flat . Hold t he flat sheet horizontally.Hold t he crumpled sheet at t he same height . Drop t hem

t oget her. Since t he sheet s are t he same, t he diff erence in

t heir falling speed is due t o t he dif f ering air resist ances of

t heir shapes.

References:


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Wood, R. W. (1992) . Science f or Kids: 3 9 Easy Engineer ing Exper iment s.

Blue Ridge Summ it ,PA: TAB Books (a division of McGraw-Hill) .


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U s ing Sp r i ng Sca les

Spring scales can be used t o measure f orces.

Mat erials:

t wo spring scales

s t r ing

board

several books

wood block wit h nail or screw for at t aching spring scale

wax paper

sandpaper

bat h towel

a luminium fo i l

Inst ruct ions:

1. If an object does not move, the forces act ing upon it must be

balanced. If I pull on somet hing and it does not move, then an

equal and opposit e force must be act ing on it . (For example, if

I pull on a heavy piece of f urnit ure and it does not budge, t hen

the friction between it and the floor must be counteracting my

force.) To demonst rate t hese opposing for ces, t ie t he t op of a

spring scale t o an immov able object .

2. At t ach t he hook of t he spring scale t o t he hook of a second

spring scale.

3. Pull on t he second spring scale. Read t he f orce t hat is

measured by each spring scale. Are t hey t he same?

4. What would happen if you used your double spring scales t o

drag an object ? Try it . What f orce registers on each scale?

5. You can also use a spring scale t o measure fr ict ion. Make a

ramp from a board and some books.

6. At t ach a spring scale t o a block of wood. Drag t he wood up t he


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ramp by pulling on t he spring scale. What does t he scale

reg is ter?

7. Cover t he ramp wit h wax paper. Drag the block up t he covered

ramp. Is t he amount of f orce used dif f erent on t he wax paper

t han on t he plain ramp?

8. Repeat t he experiment using aluminium foil, sandpaper, a bat h

t owel, or ot her mat erials.

Which mat erials require you t o use t he most f orce in dragging t he block?

The least ?

How is the f orce used relat ed t o f r ict ion?

Make a graph of your result s.

References:

Lowery, L. (198 5) . The Every day Science Sourcebook: Ideas f or Teaching

in t he Element ary and Middle School. Palo Alt o, CA: Dale Seymour

Publicat ions.




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Paper Wing

Make an airplane wing out of paper.

Mat erials:

paper

scissors

met r ic ru ler

tape

Inst ruct ions:

1. Cut a sheet of paper about 1 0 cm by 20 cm.

2. Fold t he paper in half widt hwise.

3. Tape one short edge of t he paper about 3 cm above the ot her

short edge.

4. Insert t he ruler inside t he paper wing, wit h t he edge of t he

ruler rest ing against t he fold.

5. Hold t he ruler so t hat t he folded edge of t he wing is f acing you.

Blow hard at t he fold.

What happens? Why?

References:




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Ba l lo o n i n a Bo t t l e

Can you blow up a balloon in a bot t le?

Mat erials:

t wo one- l it er soda bot t les

t wo balloons wit h wide openings

nai l

Inst ruct ions:

1. Using t he nail, punch a small hole in one of t he bot t les, about

f ive centimeters from the bottom.

2. Insert an uninf lated balloon int o each bot t le, st ret ching t heopening of t he balloon over t he mout h of t he bot t le. Do you

t hink wou wil l be able t o blow up t he balloons inside t he

bot t les? Why or why not ?

3. Try blowing up t he balloons. What happens? Can you explain

your result s in t erms of air pressure?

4. Now ful ly inf lat e t he balloon inside t he bot t le wit h t he hole.

When you are done, hold your finger over the opening. What do

you t hink will happen when you remove your finger? Try it .

5. Inflat e t he balloon again, holding your f inger over t he hole.

Now f i l l the inf lat ed bal loon wit h wat er. If y ou remove your

f inger, do you t hink t he increase in air pressure as t he air

ent ers the hole will be st rong enough t o push t he wat er out?

Try it and see.

Reference:




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( Dist ribut ed by Sargent -Welch Scient ific Company, 1 -8 00 -SARGENT).


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Bl o w i n g t h r o u g h a F u nn e l

Blowing t hrough each end of a f unnel can demonst rat e diff erences in air

pressure.

Mat erials:

clean funnel

candle in candle holder

matches

Inst ruct ions:

1. Make sure that t he candle is f irmly secured in t he candle

holder or is ot herwise st able.

2. Light t he candle.

3. Put t he narrow end of t he funnel in your mout h and t ry t o blow

out t he candle. Can you do it ?

4. Now t urn t he f unnel around. Try t o ext inguish t he candle by

blowing t hrough t he wide end. What happens?

Which posit ion made it easier t o blow out t he candle? Why?

For each posit ion of t he funnel, where was t he pressure highest ? Where

was i t lowest ?

Reference:

Wood, R. W. (1992) . Science f or Kids: 3 9 Easy Engineer ing Exper iment s.



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The Burp ing Funne l

Mat erials:

Erlenmayer f lask

one-hole st opper to f i t f lask

long-st emmed glass f unnel

f ood coloring

glycerin

Inst ruct ions:

1. Careful ly insert t he st em of t he funnel int o t he hole of t he

st opper. It is best t o coat t he end of t he st em wit h glycerin

bef ore insert ing it . ( The glycerin allows t he st em t o slide in

easily wit hout breaking.)

2. Insert t he st opper into t he flask so t hat t he funnel is above t he

f lask. Pour colored wat er int o t he f unnel.

3. The f unnel should begin t o burp. If t he wat er does not f low

down into t he f lask, t ry j iggling t he f lask t o get i t st art ed. If

t he water f lows smoot hly wit hout burping, make sure t hat t he

st opper is making a t ight seal wit h t he flask.

Why does t he can burp instead of f lowing smoot hly?

At each stage of the burp, is the pressure greater inside the flask or

outside?

A similar phenomenon can be seen when pouring liquid out of any cont ainer

wit h a narrow opening. Why do people punch t wo holes on opposit e sides

of large juice cans?


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The Heavy Paper

Mat erials:

ru ler

large sheet of paper

table or other f lat surface

Inst ruct ions:

1. Posit ion t he ruler so t hat about t wo-t hirds of i t is on t he t able

and the remaining t hird is hanging over t he edge.

2. Lay t he sheet of paper on the t able over t he ruler.

3. Hit t he overhanging end of t he ruler wit h a downward mot ion.

Can you make t he paper f ly int o t he air?

What happens if you change t he size of t he paper?

Explain your result s in t erms of pressure.

Reference:




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L i f t i n g Cu p s

Can you l if t t wo cups without t ouching t hem?

Mat erials:

t wo cups

balloon

Inst ruct ions:

1. Can you f igure out a way t o l if t t he cups wit hout t ouching

t hem, using only t he balloon?

2. Heres t he answer: Place t he cups on t heir sides, about 2

centimeters apart, with the rims facing each other.

3. Place the uninflat ed balloon bet ween t he t wo cups. Slowly

blow up t he balloon. It will push t he cups apart .

4. Lift t he balloon. The cups will come t oo.

Why do t he balloons st ick t o t he cup?

(Hint : What happens t o t he air inside t he cup as you inflat e t he balloon?

How does t he air pressure inside t he cup change when you pull on t he

balloon?)

Reference:


Spark your Imaginat ion. Wichit a, Kansas: Jakes At t ic Product ions.( Dist ribut ed by Sargent -Welch Scient ific Company, 1 -8 00 -SARGENT) .


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Pr e s su r e B o t t l e

Mat erials:one-lit er soda bot t le wit h t hree small holes punched in it as shown

bot t le cap

t hree push pins

large shallow pan t o hold bot t le

Inst ruct ions:

1. Fi ll t he bot t le with wat er but do not cap i t . Remove t he

middle pin. Does any wat er come out ?

2. Now screw t he l id on t ightly. Does water come out now?

3. Loosen t he cap. What happens? Why does capping the bot t le

make a dif f erence?

4. Replace t he middle pin and ref ill t he bot t le. Cap it . Take outt he middle pin and squeeze t he bot t le. Does water come out ?

5. Squeeze hard and then lightly. How does t he st rength of your

squeeze aff ect t he st ream of wat er?


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6. When you stop squeezing, watch t he hole carefully. What

happens? Why do you think t his happens?

7. Replace t he middle pin and refi l l t he bot t le. Remove all t hree

pins and t he cap. How do t he t hree st reams of water diff er?

Make a sket ch of t he bot t le and t he st reams below.

8. Wat ch your bot t le for a few minut es. As t he level of t he wat er

goes down, how do the st reams change? Why do you t hink t his

happens?

9. Replace all t hree pins and ref i l l t he bot t le. Cap it . Now

remove any two of t he pins. What happens?

10. Now t ry t wo ot her pins. Try all of t he combinat ions. Is t here

any diff erence bet ween t aking t he t op and bot t om pins out and

t aking t he middle and bot t om pins out ? Describe t he

diff erence. What do you t hink causes t his dif ference?

Adapt ed f rom an act ivit y by Michael Fox, Shady Hill School

Reference:

Wood, R. W. (1992) . Science for Kids: 39 Easy Engineering Experiment s.



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B al a n c i n g Me t e r St i c k

Mat erials:

meter s t ick

t wo rubber bands

several ident ical met al washers

t wo paper cl ips

Inst ruct ions:

1. Hold t he met er st ick horizont ally, rest ing i t on your t wo

outstretched index fingers, one at each end of the stick.

2. Slowly move your t wo fingers t owards each other. When your

f ingers meet , t he ruler should st i l l balance. This point on t hemet er st ick is cal led i t s center of gravity. Not ice t hat your

fingers do not move smoot hly. The f inger t hat is closer to t he

cent er of gravit y has more weight on it, so it does not move as

easily. Once t he ot her finger get s closer, t he sit uat ion

reverses and t he first f inger begins t o slide.

3. Now wrap a rubber band around each end of t he met er st ick.

Bend each paper clip int o an s shape and hang one clip from

each rubber band.

4. Try f inding t he cent er of gravit y of your meter st ick again. Is

it in t he same place (or almost t he same place)? Why or why

not?

5. Now place washers on t he hooks. If you add t he same number

of washers, does t he met er st ick balance in t he same place?

What happens if you use dif f erent numbers of washers?

6. Try moving one of t he rubber bands closer t o t he cent er. What

happens?


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7. Can you f ind a way t o place t he rubber bands so t hat diff erent

numbers of washers on each side will balance?

Can you figure out the relationship between the position of the rubber

band (dist ance from t he center) and t he number of washers ( f orce)?

How is t his related t o t orque?

Can you explain what is happening in t erms of t he equat ions for t orque?

Reference:




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T o r q u e T es t e r

Mat erials:

t wo-inch-wide plast ic pipe, one piece t hree feet long and t wo pieces

one f oot long each

T jo in t t o f i t t he p ipe

plast ic pipe adhesive

six threaded eye bolts with nuts

e lec t r i c d r i ll

o ld window sash weight or ot her t wo-t o-f ive pound weight

snap hook for at t aching weight

Inst ruct ions:

1. Glue t he pipe pieces and t he T joint t ogether t o f orm a T.

2. Dril l six evenly spaced holes in t he longest sect ion of pipe,

put t ing t he first and t he sixt h holes near t he ends of t he pipe.

3. Put an eye bolt in each hole, securing it wit h a nut .

4. At t ach t he snap hook t o t he weight.

5. Use t he snap hook to at t ach t he weight t o t he eye bolt nearest

t he T handle. Grasp t he handles ( t he short er pipe sect ions) and

l i f t the bar.

6. Now t ry moving t he weight t o another eye bolt and li f t ing t he


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bar. What happens?

7. Try t he ot her posit ions.

Which posit ion is easiest ? Which is hardest :?

Can you explain what is happening in t erms of t he equat ions for t orque?

Reference:


Spark your Imaginat ion. Wichit a, Kansas: Jakes At t ic Product ions.( Dist ribut ed by Sargent -Welch Scient ific Company, 1 -8 00 -SARGENT) .


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To rque Too l s

Many tools make use of t orque. Can you ident ify how t orque is being used

in t he fo l lowing t ools?

Mat erials:

wrenches of v arious sizes and t ypes

nut s and bolt s t hat f i t t he wrenches

a board wit h holes for at t aching bolt s (and screws)

As many of t hese t ools as possible/ pract ical:screwdriv ers and screws

hammer

shovel

spade

scissors

punch-t ype can opener

nut cracker

garl ic press

st apler

tongs

Inst ruct ions:

1. Try using t he wrenches t o loosen and t ighten t he nut s. Does

t he amount of f orce you apply make a diff erence? Does t he

lengt h of t he wrench handle make a diff erence?


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2. Try the ot her t ools. How do t hey involve t orque?

Try drawing a f orce diagram for one of t he ot her t ools, similar t o t he

wrench diagram above.

Reference:

Rogers, C., et al. Toy ing wit h Torque: A Teacher s Guide.


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Qu a r t e r F l i c k

If you shoot a bullet out of a gun and, at t he same moment , drop anot her

bullet , which bullet wi ll hi t t he ground f i rs t? ( In the int erest s of safet y

and pract icalit y, wel l answer t his quest ion using quart ers.)

Mat erials:

t wo quart ers

a t able

Inst ruct ions:

1. Posit ion bot h quart ers so that t hey are overhanging t he edge of

the table, a few centimeters apart.

2. At t he same moment , f l ick one of t he quart ers hard with t he

f ingers of one hand while gent ly pushing t he ot her quart er off

t he edge of t he t able wit h the ot her hand. (This may require a

l it t le prac t ice. )

3. Not ice which quart er hits t he ground first . Repeat t he

experiment several t imes so t hat you are fairly sure of yo ur

result s.

Which quart er hit s t he ground f irst? Is t his what you expect ed?

Remember t hat velocit y has t wo components, speed and direct ion. In t his

experiment , we are comparing t he velocit y of t he two coins in t he

downward direct ion only. ( Clearly, t he f licked coin has a great er

horizont al velocit y.) What f orce is causing t he change in vert ical velocit y

of each quart er?

Is t he downward f orce act ing on each quart er t he same?

What does t hat imply about t heir downward velocit ies?


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S n a i l T r a i l s

Just how fast ( or slow!) does a snail crawl? Remember t hat

velocit y has t wo component s, speed and direct ion. In t his experiment , we

will t ry t o measure t he speed of snails.

To f ind t he average speed of a snail, you need t o know t he t ime it

spent crawling and t he dist ance it crawled. Measuring t he t ime is easy.

Measuring t he dist ance is harder-- snails rarely crawl in st raight l ines.

Mat erials:

land snails

glass or plast ic plat es

wax pencils

clock wit h second hand or st op wat ch

meter st icks t r ing

Inst ruct ions:

1. Gent ly place t he land snail on a glass plat e. Hold t he glass

plat e horizont ally in one hand. Wait unt i l t he snail set t les

down and begins t o crawl.

2. St art t iming t he snai l . As t he snai l crawls, t race his

movement s on t he underside of t he glass plat e wit h a wax

pencil.

3. Aft er t wo minut es (or some other reasonable amount of t ime),

st op t iming and t racing. Take t he snail off t he galss plat e.

4. Lay t he st ring on t op of y our wax t rail, f ollowing each curve as

closely as possible. When you are done, cut t he st ring at t he

end of t he trai l.

5. Your st ring should now be t he same dist ance long as t he snails

t rail. Measure t he st ring using t he met er st ick.

6. To find t he snail s speed, divide t he dist ance it crawled by t he

amount of t ime it spent crawling.


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7. Figure out t he unit s t hat you used t o report your snails speed.

( Miles per hour? Cent imet ers per second?)

How fast did your snail go? How does it s speed compare wit h t hat of t he

ot her snai ls?

Make a bar graph showing t he speeds of t he snails. What is the average

speed fo r all t he snails?

Mat erials:

Snai ls are avai lable f rom Connect icut Valley Biological ( 1 -8 0 0 -6 2 8 -7748) or other biological supply companies.

Documents

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