Free fall and motion

7/30/2019 Free fall and motion

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1.3c Free fall and motion graphs

We

Are

Learning

To

•Acceleration due to gravity, g; detailed experimental methods of

measuring g are not required.

•Terminal speed.

•Representation by graphical methods of uniform and non-uniform

acceleration; interpretation of velocity-time and displacement-time

graphs for uniform and nonuniform acceleration; significance of

areas and gradients.



Starter

Recap gravity – lots of demos!



On the Earth gravity pulls with a force of 10 Newtons

for every kilogram (10 N/kg).

Weight (N) = mass (kg) x gravitational field strength (N/kg)

A bag of sugar

has a mass of

1kg. The

Earth’s gravitypulls it with a

force of 10N



Galileo Galilei

1564 - 1642

On Earth a free-falling object has an acceleration

of 9.8 ms-2

a = g



water

air

Self sealingneedles

Heavytennis ball

Normaltennis ball

Galileo – Tennis Balls

Note: Ask students what will happen when you drop two tennis balls. Both hit thefloor as expected. Then pass the tennisballs to a student. One is heavier than theother. One tennis ball has been filled withwater. This will challenge their thinking

that heavier objects fall faster.



Note: Stand on a desk and ask students to observe which object hitsthe ground first, (1) two oranges (2) an orange and a grape. The orangeand the grape should hit the ground at the same time.

Misconception: heavier objects fall faster than lighter objects.

Galileo – Orange and Grape



Note: get two pieces of paper, crumple up the one piece and then drop themboth at the same time. The crumpled up paper hits the floor first eventhough its got the same mass as the other! Why?

Galileo – Paper



Note: Cut out a circular piece of paper slightly smaller than a real coin. Dropthem both and the coin will hit the floor first (the air resistance on the coin

takes longer to build up, so it reaches a higher terminal velocity). Then drop thecoin and the paper coin together (paper coin above real coin), they both hit theground at the same time. Why?

Galileo – Coin and Paper Trick



Coin and Paper explanation

• Before they fall:The coin has a larger weight, but they start

accelerating at the same rate.

• Shortly after drop:The paper quickly reaches its terminal velocity

The coin remains accelerating

• Just before the hit the

ground:The air resistance on the coin has increased, but

It is still accelerating!



Note: Invert the tube containing the coin and feather (with air in first).Then use the vacuum pump to extract the air. Invert again and thefeather falls at the same rate as the coin.

Galileo – ‘Guinea and Feather’



Note: Put a 1 kg mass onto a analogue weighing scale and then jumpoff a table holding the scales. Choose a weight to give a significantdeflection of the needle. Students observe the dial on the scale as

you fall with it. The dial should move anti-clockwise throughout thefall.

Safety: Take care jumping off the table (do not allow a student to

jump off the table). Make sure that the weight and pan is securedto the balance.

Free Fall – Weighing Scales



Note: Students are asked to predict what will happen to a swinging

pendulum if it is allowed to freefall.

Hold a pendulum at an angle and then jump off a table. Studentsobserve the pendulum as it falls. The pendulum should be ‘frozen’ asit falls.

Safety: take care jumping off the table (do not allow a student to jump off the table).

Free Fall - Pendulum



Note: The sound toy has a box inside that makes a noisein the tube as the box falls inside the tube. Stand on atable and jump off carefully as you invert the toy.Observe the noise of the toy as the teacher falls.

Safety: Take care jumping off the table (do not allow astudent to jump off the table).

Free Fall – Noisy Toy



Note: Fill a bottle (with a small hole in theside) with water. Observe the stream ofwater coming out of the bottle as thebottle is dropped into a dustbin.

Free Fall – Bottle With Hole

Held

Dropped



weightlessness



Practical – finding the

acceleration due to gravity



Student practical- finding the acc

due to gravity g PHOTO

Light gate

Light gate

Vernier



Terminal Velocity

• Note – this can be skipped until Newton’s

Laws are covered – it may be clearer then!



Skydiver



Terminal VelocityConsider a skydiver:

1) At the start of his jump the airresistance is _______ so he

_______ downwards.

2) As his speed increases his airresistance will _______

3) Eventually the air resistance will be

big enough to _______ theskydiver’s weight. At this pointthe forces are balanced so hisspeed becomes ________ - this iscalled TERMINAL VELOCITY





SUVAT problems

Object fallingObject being thrown



Ground

Object falling

due to gravity

Object being thrown

upwards (against gravity)

g = +

s = +v = +

g = -

s = -

v = -

A coin as released at rest at the top of a ell It took 1 6 s to it



A coin was released at rest at the top of a well. It took 1.6 s to it

the bottom of the well. Calculate:

v = u + at

s = (u + v)t2

s = ut + ½ at2

v2 = u2 + 2as

(i) the distance fallen by the coin

(ii) its speed just before impact

v =

u = 0

s = ?

t = 1.6

a = -9.8

v = ?u = 0

s =

t = 1.6

a = -9.8

s = ut + ½ at2

s = -12.5 m(- indicates 12.5 m downwards)

v = u + at

v = -15.7ms-1

(- indicates downward velocity)



Homework

Questions page 121



C id b ll th di tl d d



Consider a ball thrown directly upwards andcaught when it returns. The ball rises to amaximum height of 2 metres

What would the following

graphs look like?

1. Distance-time2. Displacement-time

3. Velocity-time

Think about what the gradient of the

line represents on each graph and

what the area underneath the line

represents on a velocity-time graph)



Difference between a distance-time graph

and a displacement-time graph



displacement

time

Maximum height

Displacement-time graph for an object projected upwards



distance

time

Maximum height

Distance-time graph for an object projected upwards



Difference between a speed-time graph

and a velocity-time graph



velocity

time

Velocity-time graph for an object projected upwards

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Free fall and motion