Chapter 8Chapter 8

Motion and ForcesMotion and Forces

Reference Frame Reference Frame

Clues are often given by looking at Clues are often given by looking at other objects in your surroundingsother objects in your surroundings

Normally you think of walls or Normally you think of walls or signs as not moving, or as being signs as not moving, or as being stationary objectsstationary objects

When you do this you use the walls When you do this you use the walls or signs as a frame or referenceor signs as a frame or reference

Object that you assume is fixed in Object that you assume is fixed in placeplace

Reference frames also move Reference frames also move relative to each other which may relative to each other which may cause confusioncause confusion

The perception of motion depends The perception of motion depends on the observer’s frame of on the observer’s frame of referencereference

Reference FrameReference Frame

Describe the Describe the motion observed motion observed by one of the by one of the boys in the boys in the drawing, how drawing, how does the motion does the motion appear to be appear to be different to the different to the other boy?other boy?

Imagine you Imagine you are the girl are the girl observing the observing the bus, describe bus, describe the motion of the motion of each object each object that you can that you can seesee

Frame of ReferenceFrame of Reference How would the earth’s movement How would the earth’s movement

appear to astronauts?appear to astronauts? What are up and down directions What are up and down directions

to the astronauts? Why?to the astronauts? Why? What do you use as your frame of What do you use as your frame of

reference most of the time?reference most of the time?

Measuring MotionMeasuring Motion How do you describe motion taking How do you describe motion taking

place?place? To describe motion you discuss To describe motion you discuss

speedspeed Speed is the distance an object Speed is the distance an object

travels per unit of timetravels per unit of time To calculate its speed you divide To calculate its speed you divide

the distance it travels by the time the distance it travels by the time it travels it travels

Measuring MotionMeasuring Motion Speed is sometimes expressed in Speed is sometimes expressed in

kilometers per hourkilometers per hour Or meter per secondOr meter per second An objects speed doesn’t indicate An objects speed doesn’t indicate

all there is to know about its all there is to know about its motionmotion

An objects speed together with its An objects speed together with its direction of motion is called direction of motion is called velocityvelocity

Measuring MotionMeasuring Motion People often use the word speed People often use the word speed

when they mean velocitywhen they mean velocity Since a moving object always Since a moving object always

travels in some direction, velocity travels in some direction, velocity is a more precise term for is a more precise term for describing motiondescribing motion

Meteorologists use wind velocity Meteorologists use wind velocity measurements to help predict measurements to help predict weatherweather

Constant speedConstant speed A moving object that doesn’t A moving object that doesn’t

change it’s speed travels at change it’s speed travels at constant speedconstant speed

Constant speed means equal Constant speed means equal distances are covered in an equal distances are covered in an equal amount of timeamount of time

Suppose you and a friend want to Suppose you and a friend want to run around a track at constant run around a track at constant speed for half an hourspeed for half an hour

Constant speedConstant speed How can you check to see if your How can you check to see if your

speed is constant?speed is constant? Your measurement can be even Your measurement can be even

more accurate if you measure how more accurate if you measure how long it takes to travel very short long it takes to travel very short distances of equal lengthdistances of equal length

If all the times are the same they If all the times are the same they must be constant.must be constant.

How does this graph display speed?How does this graph display speed?

Why are these graphs different?Why are these graphs different? How was the motion different?How was the motion different?

Average SpeedAverage Speed Average speedAverage speed is equal to the total is equal to the total

distance of the course divided by distance of the course divided by the runner’s total timer.the runner’s total timer.

Calculating SpeedCalculating Speed Speed = Speed = Distance Distance

Time Time If a runner travels 100 m in 10 If a runner travels 100 m in 10

seconds what was his average seconds what was his average speed?speed?

Probably not constantProbably not constant Can solve for the other pieces tooCan solve for the other pieces too Distance = speed x timeDistance = speed x time Time = Time = DistanceDistance

Speed Speed

D

S T

Cover the one you’re looking for

PracticePractice A car race is 500 km long. It takes A car race is 500 km long. It takes

the winner 2.5 hours to complete the winner 2.5 hours to complete it. How was he going?it. How was he going?

It is 320 km to Las Vegas. If you It is 320 km to Las Vegas. If you average 80 km/hr, how long will it average 80 km/hr, how long will it take you to get there?take you to get there?

You are going on a trip. You You are going on a trip. You average 80 km/hr for 6 hours. How average 80 km/hr for 6 hours. How far did you go?far did you go?

VelocityVelocity Is both speed and direction.Is both speed and direction. 40 km/hr = speed40 km/hr = speed 40 km/hr west = velocity40 km/hr west = velocity Can change velocity two waysCan change velocity two ways Change speedChange speed Or change directionsOr change directions

MomentumMomentum A truck is harder to stop than a carA truck is harder to stop than a car Mass affects motionMass affects motion Momentum = mass x velocityMomentum = mass x velocity Symbol is pSymbol is p p = mvp = mv Units kg·m/sUnits kg·m/s Has direction just like velocityHas direction just like velocity

Calculating MomentumCalculating Momentum A 75 kg man is traveling 10 km/hr A 75 kg man is traveling 10 km/hr

west. What is his momentum?west. What is his momentum? A 0.25 kg ball is moving at 160 A 0.25 kg ball is moving at 160

km/hr toward home plate. What is km/hr toward home plate. What is its momentum?its momentum?

Law of Conservation of Law of Conservation of MomentumMomentum

The total amount of momentum in The total amount of momentum in a system is conserved.a system is conserved.

Add up all the momentumAdd up all the momentum Take into account the direction.Take into account the direction. Used to predict motion of cars Used to predict motion of cars

after a collisionafter a collision

Change in VelocityChange in Velocity Each time you take a step you are Each time you take a step you are

changing the velocity of your body.changing the velocity of your body. You are probably most familiar You are probably most familiar

with the velocity changes of a with the velocity changes of a moving bus or car.moving bus or car.

The rate at which velocity changes The rate at which velocity changes occur is called acceleration.occur is called acceleration.

Acceleration= Acceleration= final velocity- starting final velocity- starting velocityvelocity

timetime

Change in velocity = final Change in velocity = final – – starting starting

velocity velocity velocity velocity

Acceleration= Acceleration= change in velocitychange in velocity

timetime

PositivePositive acceleration acceleration

Negative Negative accelerationacceleration

AccelerationAcceleration Any change in velocity is Any change in velocity is

acceleration, even if the speed of acceleration, even if the speed of the object remains the same.the object remains the same.

When ever an object changes how When ever an object changes how it moves, the velocity changes.it moves, the velocity changes.

A change in direction is a change A change in direction is a change in velocity, and acceleration.in velocity, and acceleration.

Motion in a circleMotion in a circle An object moving in a circle or a An object moving in a circle or a

curve is constantly changing curve is constantly changing direction.direction.

CentripetalCentripetal = acceleration = acceleration towards the center of the circle.towards the center of the circle.

ForceForce A push or a pullA push or a pull Can cause a change in motionCan cause a change in motion Can cause a change in velocityCan cause a change in velocity Can cause accelerationCan cause acceleration There can be no acceleration There can be no acceleration

without a forcewithout a force

Net ForceNet Force Usually many forces are acting at Usually many forces are acting at

the same timethe same time Have to add up these forces to see Have to add up these forces to see

whether they add up or cancel out.whether they add up or cancel out. Balanced ForcesBalanced Forces cancel out and cancel out and

give a net force of zerogive a net force of zero Balanced forces can not cause a Balanced forces can not cause a

change in motionchange in motion Like a tug of warLike a tug of war

Balanced ForcesBalanced Forces

Unbalanced ForcesUnbalanced Forces The forces don’t cancel outThe forces don’t cancel out Cause a change in motionCause a change in motion Act as one forceAct as one force

FrictionFriction A force between two objects that A force between two objects that

opposes motionopposes motion A ball will not roll foreverA ball will not roll forever It will slow down because it rubs It will slow down because it rubs

against the groundagainst the ground The friction causes a negative The friction causes a negative

accelerationacceleration To keep a car moving at a constant To keep a car moving at a constant

speed, you need to keep applying speed, you need to keep applying a forcea force

FrictionFriction Friction can keep an object from Friction can keep an object from

movingmoving Rougher surfaces have greater frictionRougher surfaces have greater friction Smoother surfaces have less frictionSmoother surfaces have less friction Larger surface area has more frictionLarger surface area has more friction Greater weight has more frictionGreater weight has more friction Sliding friction is greater than rolling Sliding friction is greater than rolling

friction.friction.

FrictionFriction Friction affects every object on the Friction affects every object on the

earth.earth. Why we use oil and bearingsWhy we use oil and bearings Without friction you wouldn’t be Without friction you wouldn’t be

able to walk without slipping and able to walk without slipping and falling down.falling down.

Air ResistanceAir Resistance The force of the air against a moving The force of the air against a moving

objectobject Increases as the velocity of the Increases as the velocity of the

motion increasesmotion increases The size and shape of the object also The size and shape of the object also

effect the air resistanceeffect the air resistance Larger surface area more resistanceLarger surface area more resistance Car designers try to minimize itCar designers try to minimize it Overcoming air resistance uses more Overcoming air resistance uses more

fuelfuel

GravityGravity A force of attraction between objectsA force of attraction between objects Can act at a distance, they don’t Can act at a distance, they don’t

have to touchhave to touch The strength of the force depends on The strength of the force depends on

the mass of the objects and the the mass of the objects and the distancedistance

You multiply the masses togetherYou multiply the masses together Gravity is a weak forceGravity is a weak force The masses need to be large to be The masses need to be large to be

noticednoticed

GravityGravity Like the size of planetLike the size of planet And divide by the distance squaredAnd divide by the distance squared Twice as far is one quarter as Twice as far is one quarter as

strongstrong Three times is one ninth as strongThree times is one ninth as strong

Newton’s Laws of MotionNewton’s Laws of Motion An object at rest stays at rest until an An object at rest stays at rest until an

outside force causes it to move.outside force causes it to move. An object in motion continues to An object in motion continues to

move in the same direction at the move in the same direction at the same speed until a force stops it or same speed until a force stops it or changes its direction.changes its direction.

So, an object at rest will stay at rest, So, an object at rest will stay at rest, and an object in motion will remain in and an object in motion will remain in motion unless acted by an outside motion unless acted by an outside force.force.

Newton’s First Law of MotionNewton’s First Law of Motion

InertiaInertia An object at rest stays at rest until An object at rest stays at rest until

an outside force causes it to move.an outside force causes it to move. An object in motion continues to An object in motion continues to

move in the same direction until a move in the same direction until a force stops it or changes its force stops it or changes its direction.direction.

So, an object at rest will stay at So, an object at rest will stay at rest, and an object in motion will rest, and an object in motion will remain in motion unless acted by an remain in motion unless acted by an outside force.outside force.

InertiaInertia Inertia: the tendency of an object Inertia: the tendency of an object

to remain at rest or in motion until to remain at rest or in motion until acted upon by an external force.acted upon by an external force.

Friction is an outside force that Friction is an outside force that resists motion when two surfaces resists motion when two surfaces come in contact.come in contact.

The surfaces can be between two The surfaces can be between two objects or between an object and objects or between an object and air or water.air or water.

Second Law of MotionSecond Law of Motion Acceleration depends on the mass of Acceleration depends on the mass of

the object and the unbalanced force the object and the unbalanced force appliedapplied

F = m x aF = m x a more mass, harder to acceleratemore mass, harder to accelerate more force, faster accelerationmore force, faster acceleration Newton is the unit of forceNewton is the unit of force equal the force needed to change equal the force needed to change

the velocity of a 1 kg mass by 1 m/sthe velocity of a 1 kg mass by 1 m/s22

Free fallFree fall When the force of gravity is the When the force of gravity is the

only force acting on an objectonly force acting on an object If there was no air, all objects If there was no air, all objects

would fall at the same speedwould fall at the same speed The acceleration caused by gravity The acceleration caused by gravity

is 9.8 m/sis 9.8 m/s22

Called gCalled g Is the same for all objectsIs the same for all objects

WeightWeight The force of gravity on an object.The force of gravity on an object. F = m x aF = m x a W = m x gW = m x g Larger mass, larger weightLarger mass, larger weight Astronauts in orbit in the shuttle Astronauts in orbit in the shuttle

are falling with the same are falling with the same acceleration as the shuttleacceleration as the shuttle

There is apparent weightlessnessThere is apparent weightlessness Different planets different values Different planets different values

of g, so you would weigh different of g, so you would weigh different amountsamounts

How Things FallHow Things Fall

GalileoGalileo

1600’s 1600’s Studied how things fellStudied how things fell Didn’t have a good clockDidn’t have a good clock Rolled balls down an inclined planeRolled balls down an inclined plane Found that the speed increased as Found that the speed increased as

it rolled down the rampit rolled down the ramp

GalileoGalileo

t = 0t = 1 second

t = 2 seconds

t = 3 seconds

GalileoGalileo

Same things happen when Same things happen when things fallthings fall

Didn’t drop things from Tower of Pisa

FallingFalling Things accelerateThings accelerate acceleration needs a forceacceleration needs a force caused by gravitycaused by gravity Doesn’t depend on massDoesn’t depend on mass 9.8 m/s9.8 m/s22 After 1 second falling at 9.8 m/sAfter 1 second falling at 9.8 m/s After 2 seconds 19.6 m/sAfter 2 seconds 19.6 m/s 3 seconds 29.4 m/s3 seconds 29.4 m/s

FallingFalling Air resistance will increase as it Air resistance will increase as it

falls fasterfalls faster An upward force on the objectAn upward force on the object Eventually gravity will balance with Eventually gravity will balance with

air resistanceair resistance Reaches terminal velocity - highest Reaches terminal velocity - highest

speed reached by a falling object.speed reached by a falling object.

Terminal velocityTerminal velocity

Force of gravity is Force of gravity is constantconstant

air resistance increases as you speed up until the force is equal

Equal forces, no acceleration constant velocity terminal velocity

Motion in Two DirectionsMotion in Two Directions Things can move sideways and Things can move sideways and

vertically at the same timevertically at the same time If no force other than gravity acts,If no force other than gravity acts, the sideways velocity will remain the sideways velocity will remain

the samethe same The vertical velocity will changeThe vertical velocity will change Gives a curved pathGives a curved path ParabolaParabola

Motion in two directionsMotion in two directions

Third Law of MotionThird Law of Motion For every force, there is an equal For every force, there is an equal

and opposite forceand opposite force For every action there is an equal For every action there is an equal

and opposite reaction.and opposite reaction. RocketsRockets gases get pushed outgases get pushed out Rocket moves forwardRocket moves forward SkatingSkating

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