PP_Revised

Forces and Motion Forces and Motion

Prentice Hall Prentice Hall Physical Science, Physical Science, Concepts in ActionConcepts in Action, pages 354 – , pages 354 –

387; 387; http://http://

www.physicsclassroom.com/ www.physicsclassroom.com/ Class/newtlaws/newtltoc.html Class/newtlaws/newtltoc.html

Objective 1Objective 1 Define force and describe how Define force and describe how

forces affect the motion of an forces affect the motion of an object; state the SI unit for object; state the SI unit for force.force.

ForcesForces A A forceforce is a push or pull that is a push or pull that

acts on an object. acts on an object. A force can set an object at rest A force can set an object at rest

into motion, or it can accelerate into motion, or it can accelerate a moving object by changing the a moving object by changing the objectobject’’s speed or direction.s speed or direction.

The NewtonThe Newton Force is measured in Newtons, Force is measured in Newtons,

abbreviated as N. abbreviated as N. One Newton is the force that One Newton is the force that

causes a 1-kilogram mass to causes a 1-kilogram mass to accelerate at a rate of 1 meter per accelerate at a rate of 1 meter per second each second (1 m/ssecond each second (1 m/s22). ).

One Newton is equal to one One Newton is equal to one kilogramkilogrammeter per second meter per second squared (1 N = 1 kgsquared (1 N = 1 kgm/sm/s22).).

Objective 2Objective 2 List the four universal forces List the four universal forces

and describe each of these and describe each of these forces.forces.

Universal ForcesUniversal Forces Observations of planets, stars Observations of planets, stars

and galaxies strongly suggest and galaxies strongly suggest four different forces exist four different forces exist throughout the universe. throughout the universe.

These forces are known as These forces are known as universal forces. universal forces.

The four universal forces are the The four universal forces are the electromagnetic, strong nuclear, electromagnetic, strong nuclear, weak nuclear, and gravitational.weak nuclear, and gravitational.

1. Electromagnetic Forces1. Electromagnetic Forces Electric and magnetic forces are Electric and magnetic forces are

two different aspects of the two different aspects of the electromagnetic force. electromagnetic force.

Electromagnetic forceElectromagnetic force is is associated with charged particles. associated with charged particles.

Electric force and magnetic force Electric force and magnetic force are the only forces that can both are the only forces that can both attract and repel. attract and repel.

Electric ForcesElectric Forces Electric forcesElectric forces act between act between

charged objects or particles such charged objects or particles such as electrons and protons. as electrons and protons.

Objects with opposite charges – Objects with opposite charges – positive and negative – attract positive and negative – attract one another. one another.

Objects with like charges repel Objects with like charges repel one another. one another.

Magnetic ForcesMagnetic Forces Magnetic forcesMagnetic forces act on certain act on certain

metals, on the poles of magnets and metals, on the poles of magnets and on moving charges. on moving charges.

Magnets have two poles, north and Magnets have two poles, north and south. south.

Two poles that are alike repel each Two poles that are alike repel each other. other.

Two opposite poles attract each other. Two opposite poles attract each other.

2. Strong Nuclear Forces2. Strong Nuclear Forces The The strong nuclear forcestrong nuclear force is a is a

powerful force of attraction that holds powerful force of attraction that holds the nucleus together. the nucleus together.

Although this force acts over only Although this force acts over only extremely short distances, it is 100 extremely short distances, it is 100 times stronger than the electric force times stronger than the electric force of repulsion.of repulsion.

3. Weak Nuclear Forces3. Weak Nuclear Forces The weak nuclear force is weaker in The weak nuclear force is weaker in

strength than the strong nuclear strength than the strong nuclear force. force.

The The weak nuclear forceweak nuclear force is an is an attractive force that acts only over attractive force that acts only over a short range and affects all a short range and affects all particles.particles.

Your mass on different Your mass on different planetsplanets

http://www.solarviews.com/eng/edu/weight.htm



4. Gravitational Forces4. Gravitational Forces Gravitational forceGravitational force is an attractive is an attractive

force that acts between any two force that acts between any two masses. masses.

NewtonNewton’’s law of universal gravitation s law of universal gravitation states that every object in the universe states that every object in the universe attracts every other object. attracts every other object.

Gravitational forces act over large Gravitational forces act over large distances. distances.

Gravity is the weakest universal force, Gravity is the weakest universal force, but it is the most effective force over but it is the most effective force over long distances. long distances.

Objective 3Objective 3 Identify forces as vector Identify forces as vector

quantities and define net force.quantities and define net force.

ForcesForces Forces are Forces are vector quantitiesvector quantities. . That is, they have both magnitude That is, they have both magnitude

and direction. and direction. Vectors are represented by using Vectors are represented by using

arrows. arrows. The direction of the arrow represents The direction of the arrow represents

the direction of the force. the direction of the force. The length of the arrow represents the The length of the arrow represents the

strength, or magnitude, of the force. strength, or magnitude, of the force.

Combining Force VectorsCombining Force Vectors Vector addition may be used to Vector addition may be used to

combine force vectors. combine force vectors. Forces acting in the same direction add Forces acting in the same direction add

together; forces acting in opposite together; forces acting in opposite directions are subtracted. directions are subtracted.

The Pythagorean Theorem may be used The Pythagorean Theorem may be used to combine vectors acting at right to combine vectors acting at right angles.angles.

A A net forcenet force is the overall force acting is the overall force acting on an object after all the forces are on an object after all the forces are combined.combined.

Finding Net ForceFinding Net Force

Objective 4Objective 4 Distinguish Distinguish

between between balanced and balanced and unbalanced unbalanced forces. forces.

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

Balanced forcesBalanced forces are forces that are forces that combine to produce a net force of combine to produce a net force of zero. zero.

When balanced, there is no change in When balanced, there is no change in the objectthe object’’s motion. It is at rest.s motion. It is at rest.

An An unbalanced forceunbalanced force results when results when the net force acting on an object is the net force acting on an object is not equal to zero. not equal to zero.

When an unbalanced force acts on an When an unbalanced force acts on an object, the object accelerates. object, the object accelerates.

Balanced ForcesBalanced Forces

Unbalanced ForcesUnbalanced Forces

Situation ASituation A What is the net What is the net

force vertically?force vertically?20 N – 20 N = 020 N – 20 N = 0

What is the net What is the net force force horizontally?horizontally?00

Are the forces Are the forces balanced?balanced?yesyes

Situation BSituation B What is the net What is the net

force vertically?force vertically?3 N – 3 N = 03 N – 3 N = 0

What is the net What is the net force force horizontally?horizontally?5 N – 5 N = 05 N – 5 N = 0

Are the forces Are the forces balanced?balanced?yesyes

Situation CSituation C What is the net What is the net

force vertically?force vertically? 40 N – 25 N = 15 N 40 N – 25 N = 15 N

What is the net What is the net

force horizontally?force horizontally? 00

Are the forces Are the forces balanced?balanced? nono

What is the net What is the net force?force? 15 N upward15 N upward

Situation DSituation D What is the net What is the net

force vertically?force vertically? 3 N – 3 N = 03 N – 3 N = 0

What is the net What is the net force force horizontally?horizontally? 5 N leftward5 N leftward

Are the forces Are the forces balanced?balanced? nono

What is the net What is the net force?force? 5 N leftward5 N leftward

Sample ProblemSample Problem A rightward force of 24 N is A rightward force of 24 N is

applied to a 100 N object to applied to a 100 N object to move it across a rough surface at move it across a rough surface at constant velocity. The frictional constant velocity. The frictional force is 24 N. Draw a force force is 24 N. Draw a force diagram showing all forces diagram showing all forces acting on the object. Are the acting on the object. Are the forces balanced?forces balanced?

Answer: net F = 0Answer: net F = 0

Ff = 24 N Fa = 24 N

Fgrav = 100 N

Ftable = 100 N constant

velocity

Sample ProblemSample Problem Suppose the object in the above Suppose the object in the above

problem has an applied force of problem has an applied force of 36 N. Draw the new force 36 N. Draw the new force diagram. Are the forces diagram. Are the forces balanced? What is the net force?balanced? What is the net force?

Answer: net F = 12 N rightAnswer: net F = 12 N right

Ff = 24 N Fa = 36 N

Fgrav = 100 N

Ftable = 100 N

Objective 5Objective 5 Define friction and describe Define friction and describe

the four main types of the four main types of friction. friction.

FrictionFriction All moving objects are subject to All moving objects are subject to

frictionfriction, a force that opposes the , a force that opposes the motion of objects that touch as they motion of objects that touch as they move past each other. move past each other.

Friction acts at the surface where Friction acts at the surface where objects are in contact. objects are in contact.

There are four main types of friction:There are four main types of friction: static friction,static friction, sliding friction,sliding friction, rolling friction, androlling friction, and fluid frictionfluid friction. .

1. Static Friction1. Static Friction Static frictionStatic friction acts on objects acts on objects

that are not moving. that are not moving. Static friction always acts in the Static friction always acts in the

direction opposite to that of the direction opposite to that of the applied force. applied force.

2. Sliding Friction2. Sliding Friction Sliding frictionSliding friction opposes the opposes the

direction of motion of an object direction of motion of an object as it slides over a surface. as it slides over a surface.

Less force is needed to keep an Less force is needed to keep an object moving than to start it object moving than to start it moving.moving.

3. Rolling Friction3. Rolling Friction When a round object rolls across When a round object rolls across

a flat floor, both the object and a flat floor, both the object and the floor are bent out of shape. the floor are bent out of shape.

This change in shape at the point This change in shape at the point of rolling contact is the cause of of rolling contact is the cause of rolling frictionrolling friction, the frictional , the frictional force that acts on rolling objects.force that acts on rolling objects.

Rolling Friction, continuedRolling Friction, continued For a given set of materials, the For a given set of materials, the

force of rolling friction is about force of rolling friction is about 100 to 1000 times less than the 100 to 1000 times less than the force of static or sliding friction. force of static or sliding friction.

Because of this, wheeled dollies Because of this, wheeled dollies are used to move heavy objects are used to move heavy objects and ball bearings are used to and ball bearings are used to reduce friction in machines.reduce friction in machines.

4. Fluid Friction4. Fluid Friction Water and air are both fluids. Water and air are both fluids. The force of The force of fluid frictionfluid friction

opposes the motion of an object opposes the motion of an object through a fluid. through a fluid.

Fluid friction increases as the Fluid friction increases as the speed of the object moving speed of the object moving through the fluid increases.through the fluid increases.

Air ResistanceAir Resistance Fluid friction acting on an object Fluid friction acting on an object

moving through the air is known moving through the air is known as as air resistanceair resistance. .

At higher speeds, air resistance At higher speeds, air resistance can become a significant force.can become a significant force.

Falling With Air ResistanceFalling With Air Resistance As an object falls As an object falls

through air, it through air, it usually encounters usually encounters some degree of air some degree of air resistance. resistance.

Air resistance is the Air resistance is the result of collisions result of collisions of the object's of the object's leading surface leading surface with air molecules. with air molecules.

Objective 6Objective 6 Describe how EarthDescribe how Earth’’s gravity and air s gravity and air

resistance affect falling objects.resistance affect falling objects.

GravityGravity GravityGravity is a force that acts is a force that acts

between any two masses. between any two masses. Gravity is an attractive force, Gravity is an attractive force,

that is, it pulls objects together. that is, it pulls objects together. EarthEarth’’s gravitational force exerts s gravitational force exerts

a force of attraction on every a force of attraction on every other object that is near Earth.other object that is near Earth.

Gravity, continuedGravity, continued The force of gravity does not The force of gravity does not

require objects to be in contact require objects to be in contact for it to act on them. for it to act on them.

Gravity can act over large Gravity can act over large distances. distances.

EarthEarth’’s gravity acts downward s gravity acts downward toward the center of Earth. toward the center of Earth.

Gravity and Air ResistanceGravity and Air Resistance Gravity causes objects to Gravity causes objects to

accelerate downward, whereas air accelerate downward, whereas air resistance acts opposite to the resistance acts opposite to the direction of motion.direction of motion.

As objects fall to the ground they As objects fall to the ground they continue to accelerate and gain continue to accelerate and gain speed. speed.

With increasing speed comes With increasing speed comes increasing air resistance. increasing air resistance.

Terminal VelocityTerminal Velocity If an object falls for a long time, the If an object falls for a long time, the

upward force of air resistance becomes upward force of air resistance becomes equal to the downward force of gravity. equal to the downward force of gravity.

At this point, acceleration is zero and At this point, acceleration is zero and the object continues falling at a the object continues falling at a constant velocity. constant velocity.

Terminal velocityTerminal velocity is the constant is the constant velocity of a falling object when the velocity of a falling object when the force of air resistance equals the force force of air resistance equals the force of gravity. of gravity.

Gravity and Air ResistanceGravity and Air Resistance

Gravity and Air ResistanceGravity and Air Resistance Suppose that an elephant Suppose that an elephant

and a feather are dropped and a feather are dropped off a very tall building from off a very tall building from the same height at the the same height at the same time. same time.

We will assume the We will assume the realistic situation that both realistic situation that both feather and elephant feather and elephant encounter air resistance. encounter air resistance.

Which object - the Which object - the elephant or the feather - elephant or the feather - will hit the ground first? will hit the ground first?

An ExplanationAn Explanation

Warm-Up QuestionWarm-Up QuestionUniversal ForcesUniversal ForcesA. A.

ElectromagnetiElectromagneticc

B. Strong NuclearB. Strong NuclearC. Weak NuclearC. Weak NuclearD. GravitationalD. Gravitational

CharacteristicsCharacteristics1. Only forces that can both 1. Only forces that can both

attract and repel.attract and repel.2. Weakest force, but most 2. Weakest force, but most

effective over long effective over long distances.distances.

3. Affects all particles in 3. Affects all particles in nucleus.nucleus.

4. Holds neutrons and protons 4. Holds neutrons and protons together in the nucleus.together in the nucleus.

Warm-Up QuestionWarm-Up Question A rightward force is applied to a A rightward force is applied to a

60-N object to move it across a 60-N object to move it across a rough surface at constant rough surface at constant velocity. Constant velocity velocity. Constant velocity means net force = 0. The object means net force = 0. The object encounters 15 N of frictional encounters 15 N of frictional force. Draw the force diagram force. Draw the force diagram and determine the applied force.and determine the applied force.

Warm-Up Question Warm-Up Question AnsweredAnswered

Weight is downwardWeight is downward Table pushes upward Table pushes upward

with the same forcewith the same force Frictional force Frictional force

pushes backwardpushes backward Net force = 0 Net force = 0

(constant velocity)(constant velocity) Applied force = 15 NApplied force = 15 N

60 N

60 N

15 N 15 N

Prelab: Frictional ForcesPrelab: Frictional Forces1.1. What is friction?What is friction?2.2. What is the difference between What is the difference between

static and sliding friction?static and sliding friction?3.3. What effect does rolling friction What effect does rolling friction

have on an object?have on an object?4.4. Compare the strengths of static, Compare the strengths of static,

sliding and rolling friction.sliding and rolling friction.5.5. Draw a force diagram for a block Draw a force diagram for a block

that is being pulled across a table.that is being pulled across a table.

Force DiagramForce Diagram FFGravityGravity = - F = - FTableTable

FFAppliedApplied = - F = - FFrictionFriction Starting Force – Starting Force –

just starts to movejust starts to move Three trialsThree trials

Moving Force – Moving Force – moves at constant moves at constant speedspeed Three trialsThree trials

FAppliedFFriction

FGravity

FTable

Velocity

Lab: Frictional ForcesLab: Frictional Forces Effect of Type of Effect of Type of

SurfaceSurface Effect of Force Effect of Force

Pressing the Pressing the Surfaces Surfaces TogetherTogether

Effect of Rolling Effect of Rolling FrictionFriction

FAppliedFFriction

FGravity

FTable

Velocity

Lab Frictional ForcesLab Frictional Forces Start with the blue force measurer.Start with the blue force measurer. The four surfaces are: wood, vinyl, The four surfaces are: wood, vinyl,

sandpaper and cardboard.sandpaper and cardboard. Three trials with each surface with a 500 g Three trials with each surface with a 500 g

mass on top of the block.mass on top of the block. Record starting force and moving force.Record starting force and moving force. Add an extra 500 g mass, pull on wood Add an extra 500 g mass, pull on wood

side down.side down. They place wooden dowels under using 1 They place wooden dowels under using 1

500 g mass and wood side down.500 g mass and wood side down.

Objective 7Objective 7 State NewtonState Newton’’s first law of s first law of

motion and apply it to motion and apply it to physical situations.physical situations.

NewtonNewton’’s First Law of Motions First Law of Motion NewtonNewton’’s first law of motions first law of motion

states: the state of motion of an states: the state of motion of an object does not change as long as object does not change as long as the net force acting on the object is the net force acting on the object is zero. zero.

Unless an unbalanced force acts, an Unless an unbalanced force acts, an object at rest remains at rest, and an object at rest remains at rest, and an object in motion remains in motion object in motion remains in motion with the same speed and direction.with the same speed and direction.

InertiaInertia NewtonNewton’’s first law of motion is s first law of motion is

sometimes called the sometimes called the law of law of inertiainertia..

InertiaInertia is the tendency of an is the tendency of an object to resist a change in object to resist a change in motion.motion.

The MotorcyclistThe Motorcyclist

The Car and the WallThe Car and the Wall

Activity, pages 357 - 359Activity, pages 357 - 359FigureFigure Is Net Force Is Net Force

0?0? Effect on MotionEffect on Motion

2A2A

2B2B

33

5A5A

5B5B



2A2A YesYes

2B2B

33

5A5A

5B5B



2A2A YesYes No motionNo motion

2B2B

33

5A5A

5B5B




2B2B YesYes

33

5A5A

5B5B




2B2B YesYes No motionNo motion

33

5A5A

5B5B





33 YesYes

5A5A

5B5B





33 YesYes No motionNo motion

5A5A

5B5B






5A5A YesYes

5B5B







5B5B







5B5B NoNo







5B5B NoNo Potted tree Potted tree acceleratesaccelerates

Objective 8Objective 8 State NewtonState Newton’’s second law of s second law of

motion and apply it to physical motion and apply it to physical situations. situations.

NewtonNewton’’s Second Law of s Second Law of MotionMotion

According to According to NewtonNewton’’s second s second law of motionlaw of motion, the acceleration , the acceleration of an object is equal to the net of an object is equal to the net force acting on it divided by the force acting on it divided by the objectobject’’s mass. s mass.

Objective 9Objective 9 Use NewtonUse Newton’’s second law of s second law of

motion to calculate acceleration, motion to calculate acceleration, force and mass force and mass

Second Law of MotionSecond Law of Motion

The acceleration of an object is The acceleration of an object is always in the same direction as the always in the same direction as the net force. net force.

Second Law, continuedSecond Law, continued NewtonNewton’’s second law also applies s second law also applies

when a net force produces an when a net force produces an acceleration that reduces the speed. acceleration that reduces the speed.

This is the principle used by This is the principle used by automobile seat belts. automobile seat belts.

In a collision, the seat belt applies a In a collision, the seat belt applies a force that opposes a passengerforce that opposes a passenger’’s s forward motion. forward motion.

This force decelerates the passenger This force decelerates the passenger in order to prevent serious injury. in order to prevent serious injury.

Sample ProblemSample Problem A sailboat and its crew A sailboat and its crew

have a combined have a combined mass of 655 kg. If the mass of 655 kg. If the sailboat experiences a sailboat experiences a net force of 895 N net force of 895 N pushing it forward, pushing it forward, what is the sailboatwhat is the sailboat’’s s acceleration?acceleration?

1.37 m/s1.37 m/s22

Sample ProblemSample Problem Zookeepers lift a stretcher that holds Zookeepers lift a stretcher that holds

a sedated lion. The total mass of the a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion and stretcher is 175 kg, and the lionlion’’s upward acceleration is 0.657 s upward acceleration is 0.657 m/sm/s22. What is the net force necessary . What is the net force necessary to produce this acceleration of the to produce this acceleration of the lion and the stretcher?lion and the stretcher?

115 N115 N

Sample ProblemSample Problem A child is playing A child is playing

with some blocks with some blocks and gets mad. If a and gets mad. If a block pushed with a block pushed with a force of 13.5 N force of 13.5 N accelerates at 6.5 accelerates at 6.5 m/sm/s22 to the left, to the left, what is the mass of what is the mass of the block?the block?

2.1 kg2.1 kg

Objective 10Objective 10 Relate the mass of an object to Relate the mass of an object to

its weight.its weight.

Mass and WeightMass and Weight Mass and weight are not the Mass and weight are not the

same. same. MassMass is a measure of the inertia is a measure of the inertia

of an object. of an object. WeightWeight is a measure of the force is a measure of the force

of gravity acting on an object. of gravity acting on an object.

WeightWeight An objectAn object’’s weight is the product of s weight is the product of

the objectthe object’’s mass and the s mass and the acceleration due to gravity acting on acceleration due to gravity acting on it.it.

Sample ProblemSample Problem How much How much

does a 5.0-kg does a 5.0-kg puppy weigh puppy weigh on Earth?on Earth?

49 N49 N

Sample ProblemSample Problem A bag of sugar A bag of sugar

weighs 22 N. weighs 22 N. What is its What is its mass?mass?

2.2 kg2.2 kg

Sample ProblemSample Problem What is the weight What is the weight

of the same bag of of the same bag of sugar on the moon, sugar on the moon, where the where the acceleration due to acceleration due to gravity is one-sixth gravity is one-sixth that on Earth?that on Earth?

3.6 N3.6 N

Warm-Up QuestionWarm-Up Question A beach ball is left in the bed of a A beach ball is left in the bed of a

pickup truck. Describe what happens pickup truck. Describe what happens to the ball when the truck to the ball when the truck accelerates forward.accelerates forward.

Answer to Warm-Up Answer to Warm-Up QuestionQuestion

A beach ball will tend to stay at rest A beach ball will tend to stay at rest when the truck accelerates forward.when the truck accelerates forward.

The ball will move backward relative The ball will move backward relative to the truck.to the truck.

Warm-Up QuestionWarm-Up Question A duck is on a scooter A duck is on a scooter

accelerating at 2.0 m/saccelerating at 2.0 m/s22. If the . If the duck and scooter have a duck and scooter have a combined mass of 8.0 kg, what combined mass of 8.0 kg, what net force acts on the duck and net force acts on the duck and scooter?scooter? 16 N16 N

Where did this force come from?Where did this force come from? The duckThe duck’’s foot pushes backward s foot pushes backward

on the ground and the ground on the ground and the ground pushes the duckpushes the duck’’s foot forward with s foot forward with an equal and opposite force (3an equal and opposite force (3rdrd law).law).

Quiz: Forces and MotionQuiz: Forces and Motion Get out a sheet of Get out a sheet of

paper.paper. Place your name at Place your name at

the top of the the top of the page.page.

Number it from 1-Number it from 1-15.15.

You may use a You may use a calculator if you calculator if you have one in class have one in class today.today.

Objective 11Objective 11 Explain how Explain how

action-action-reaction reaction forces are forces are related related according to according to NewtonNewton’’s s third law of third law of motion.motion.

NewtonNewton’’s Third Law of s Third Law of MotionMotion

Forces always exist in pairs. Forces always exist in pairs. NewtonNewton’’s third law of motions third law of motion

states: whenever one object exerts states: whenever one object exerts a force on a second object, the a force on a second object, the second object exerts an second object exerts an equal and equal and oppositeopposite force on the first object. force on the first object.

These two forces are called These two forces are called actionaction and and reaction forcesreaction forces. .

Third Law, continuedThird Law, continued When you kick a soccer ball with your When you kick a soccer ball with your

foot, you notice the effect of the force foot, you notice the effect of the force exerted by your foot on the ball. exerted by your foot on the ball.

The ball experiences a change in motion. The ball experiences a change in motion. But this is not the only force present. But this is not the only force present.

The soccer ball exerts an equal but The soccer ball exerts an equal but opposite force on your foot. opposite force on your foot.

The force exerted on the ball by your The force exerted on the ball by your foot is the foot is the action forceaction force, and the force , and the force exerted on your foot by the ball is the exerted on your foot by the ball is the reaction force. reaction force.

Third Law, continuedThird Law, continued Note that the action and reaction Note that the action and reaction

forces are applied to forces are applied to different different objectsobjects. .

These These forces are equal and forces are equal and oppositeopposite, but this is not a case of , but this is not a case of balanced forces because balanced forces because two two different objectsdifferent objects are involved. are involved.

The action force acts on the ball and The action force acts on the ball and the reaction force acts on the foot. the reaction force acts on the foot.

Action-Reaction ForcesAction-Reaction Forces Identify at least six action-reaction Identify at least six action-reaction

pairs in the picture below.pairs in the picture below.

Objective 12Objective 12 Calculate the momentum of an Calculate the momentum of an

object and describe what happens object and describe what happens when momentum is conserved when momentum is conserved during a collision.during a collision.

MomentumMomentum A slow moving bicycle is easier A slow moving bicycle is easier

to stop than a fast moving one. to stop than a fast moving one. Also, a slow moving bicycle is Also, a slow moving bicycle is

easier to stop than a car easier to stop than a car traveling at the same speed. traveling at the same speed.

Increasing either the speed or Increasing either the speed or mass of an object makes it mass of an object makes it harder to stopharder to stop..

Momentum, continuedMomentum, continued A moving object has a property A moving object has a property

called momentum that is related called momentum that is related to how much force is needed to to how much force is needed to change its motion. change its motion.

MomentumMomentum is the product of an is the product of an objectobject’’s mass and its velocity. s mass and its velocity.

Momentum, continuedMomentum, continuedmomentum = mass x velocitymomentum = mass x velocity

The unit for momentum is The unit for momentum is kgkgm/s. m/s.

Momentum is a vector quantity Momentum is a vector quantity (it has direction) because (it has direction) because velocity has a direction. velocity has a direction.

Sample ProblemSample Problem Calculate the Calculate the

momentum of a momentum of a 6.00-kg bowling 6.00-kg bowling ball moving at ball moving at 10.0 m/s down 10.0 m/s down the alley.the alley.

60.0 kg60.0 kgm/s down m/s down the alleythe alley

Sample ProblemSample Problem Calculate the Calculate the

momentum of momentum of a 48.5-kg a 48.5-kg passenger on passenger on a train stopped a train stopped on its tracks.on its tracks.

0 kg0 kgm/sm/s

Sample ProblemSample Problem The train starts to The train starts to

move eastward with move eastward with a constant velocity a constant velocity of 72 m/s. A baby of 72 m/s. A baby on the train has a on the train has a momentum of 360 momentum of 360 kgkgm/s. What is the m/s. What is the mass of the baby?mass of the baby?

5.0 kg5.0 kg

Sample ProblemSample Problem A 135-kg A 135-kg

ostrich has a ostrich has a momentum of momentum of 2190 kg2190 kgm/s m/s north. What is north. What is its velocity?its velocity?

16.2 m/s north16.2 m/s north

Conservation of MomentumConservation of Momentum Imagine that two cars of different Imagine that two cars of different

masses and traveling with different masses and traveling with different velocities collide head on. velocities collide head on.

Momentum can be used to predict Momentum can be used to predict the motion of the cars after the the motion of the cars after the collision. collision.

This is because, in the absence of This is because, in the absence of outside influences, the momentum outside influences, the momentum is conserved. is conserved.

Conservation of Momentum, Conservation of Momentum, contcont’’d.d.

In physics, the word In physics, the word conservationconservation means that something has a means that something has a constant value. constant value.

That is, conservation of That is, conservation of momentum means that momentum means that momentum does not increase or momentum does not increase or decrease. decrease.

Closed SystemsClosed Systems Momentum is conserved when Momentum is conserved when

the objects are part of a the objects are part of a closed closed systemsystem. .

A closed system means other A closed system means other objects and forces cannot enter objects and forces cannot enter or leave a system. or leave a system.

Objects within the system can Objects within the system can exert forces on one another. exert forces on one another.

Law of Conservation of Law of Conservation of MomentumMomentum

According to the According to the law of law of conservation of momentumconservation of momentum, if , if no net force acts on a system, no net force acts on a system, then the total momentum of the then the total momentum of the system does not change. system does not change.

In a closed system, the loss of In a closed system, the loss of momentum of one object equals momentum of one object equals the gain of momentum of another the gain of momentum of another object – momentum is conserved.object – momentum is conserved.

Conservation of Momentum, Conservation of Momentum, contcont’’d.d.

In the example above, the total In the example above, the total momentum of the two cars before momentum of the two cars before the collision is the same as the the collision is the same as the total momentum after the total momentum after the collision. collision.

This is true if the cars bounce off This is true if the cars bounce off each other or get tangled each other or get tangled together.together.

Conservation of Momentum, Conservation of Momentum, contcont’’d. d.

Cars can bounce off each other Cars can bounce off each other to move in opposite directions. to move in opposite directions.

If the cars stick together after a If the cars stick together after a head-on collision, the cars will head-on collision, the cars will continue in the direction of the continue in the direction of the car that originally has the car that originally has the greater momentum. greater momentum.

Car Car ““Rear EndsRear Ends”” Truck Truck

Truck Truck ““Rear EndsRear Ends”” Car Car

Head-On CollisionHead-On Collision

Warm-Up QuestionWarm-Up Question The pairs of forces referred to in The pairs of forces referred to in

NewtonNewton’’s third law s third law are equal in __________;are equal in __________;opposite in __________; andopposite in __________; andact on __________ __________.act on __________ __________.

strengthdirection

different objects

Warm-Up QuestionWarm-Up Question For every force there For every force there

exists an equal and exists an equal and opposite force. If the opposite force. If the action force is action force is considered to be that considered to be that of the earth pulling of the earth pulling down on the ball, can down on the ball, can you identify the you identify the reaction force?reaction force?Ball pulling up on earthBall pulling up on earth

Warm-Up QuestionWarm-Up Question If a truck and a car have a head-on If a truck and a car have a head-on

collision, which vehicle will collision, which vehicle will experience the greater impact force?experience the greater impact force? The truckThe truck The carThe car Both the sameBoth the same It depends on other factorsIt depends on other factors

Warm-Up QuestionWarm-Up Question Jocko, who has a Jocko, who has a

mass of 80 kg and mass of 80 kg and stands at rest on stands at rest on ice, catches a 20-kg ice, catches a 20-kg ball that is thrown ball that is thrown to him at 5 m/s. to him at 5 m/s. How fast do Jocko How fast do Jocko and the ball move and the ball move across the ice?across the ice?

500 km-m/s500 km-m/s

Warm-Up QuestionWarm-Up Question The law of conservation of momentum The law of conservation of momentum

states thatstates thata. In a closed system, the total momentum a. In a closed system, the total momentum

of all objects equals zero.of all objects equals zero.b. In a closed system, the loss of b. In a closed system, the loss of

momentum of one object is less than the momentum of one object is less than the gain in momentum of another object.gain in momentum of another object.

c. In a closed system, the loss of c. In a closed system, the loss of momentum of one object is greater than momentum of one object is greater than the gain in momentum of another object.the gain in momentum of another object.

d. In a closed system, the loss of d. In a closed system, the loss of momentum of one object equals the gain momentum of one object equals the gain of momentum of another object.of momentum of another object.

Lab: Investigating a Balloon Lab: Investigating a Balloon JetJet

Blow up the balloon to Blow up the balloon to the same length for the same length for all trials. all trials.

Observe the motion of Observe the motion of a balloon as it moves a balloon as it moves across the room. across the room.

Identify the action and Identify the action and reaction forces.reaction forces.

Use NewtonUse Newton’’s second s second and third laws to and third laws to explain the motion explain the motion you observe.you observe.

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