Forces & Forces & MotionMotionChapter 12Chapter 12

ForceForce

ForceForce a push or pull that one body exerts on anothera push or pull that one body exerts on another What forces are being What forces are being

exerted on the football?exerted on the football?

Fkick

Fgrav

Measuring ForcesMeasuring Forces

Forces are measured Forces are measured in newtons in newtons

(kg (kg .. m/s m/s22).). Forces are measured Forces are measured

using a spring scale.using a spring scale.

ForceForce Balanced ForcesBalanced Forces

forces acting on forces acting on an object that are an object that are opposite in opposite in direction and direction and equal in sizeequal in size

no change in no change in velocityvelocity

ForceForce Net ForceNet Force

unbalanced forces that are not opposite unbalanced forces that are not opposite and equaland equal

velocity changes (object accelerates)velocity changes (object accelerates)

Ffriction

W

Fpull

Fnet

NN

ConcepTest 1ConcepTest 1

TRUE or FALSE?TRUE or FALSE?

The object shown in the diagram must be at rest The object shown in the diagram must be at rest since there is no net force acting on it.since there is no net force acting on it.

FALSE! A net force does not FALSE! A net force does not cause motion. A net force causes cause motion. A net force causes a a changechange in motion, or acceleration. in motion, or acceleration.

Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.

ConcepTest 2ConcepTest 2You are a passenger in a car and not wearing your You are a passenger in a car and not wearing your seat belt. seat belt.

Without increasing or decreasing its speed, the car Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself makes a sharp left turn, and you find yourself colliding with the right-hand door. colliding with the right-hand door.

Which is the correct analysis of the situation? ...Which is the correct analysis of the situation? ...

ConcepTest 2ConcepTest 21. Before and after the collision, there is a rightward 1. Before and after the collision, there is a rightward

force pushing you into the door. force pushing you into the door.

2. Starting at the time of collision, the door exerts a 2. Starting at the time of collision, the door exerts a leftward force on you. leftward force on you.

3. both of the above 3. both of the above

4. neither of the above4. neither of the above

2. Starting at the time of collision, the door 2. Starting at the time of collision, the door exerts a leftward force on you.exerts a leftward force on you.

FrictionFriction

FrictionFriction force that opposes motion between 2 force that opposes motion between 2

surfacessurfaces depends on the:depends on the:

types of surfacestypes of surfaces force between the surfacesforce between the surfaces

FrictionFriction

Four Types of FrictionFour Types of FrictionStatic FrictionStatic Friction – force that acts on objects that – force that acts on objects that

are not moving. (Couch Potato)are not moving. (Couch Potato)Sliding FrictionSliding Friction - force that opposes the - force that opposes the

direction of motion of an object as it slides direction of motion of an object as it slides over a surface. (Ice skating or bobsledding)over a surface. (Ice skating or bobsledding)

Rolling FrictionRolling Friction – friction force that acts on – friction force that acts on rolling objects. (Rollerblading)rolling objects. (Rollerblading)

Fluid FrictionFluid Friction – force that opposes the motion – force that opposes the motion of an object through a fluid. (Planes flying or of an object through a fluid. (Planes flying or submarines traveling)submarines traveling)

FrictionFriction

Friction is greater...Friction is greater... between rough surfacesbetween rough surfaces when there’s a greater force when there’s a greater force

between the surfaces between the surfaces (e.g. more weight)(e.g. more weight)

GravityGravity

GravityGravity

force of attraction between any two force of attraction between any two objects in the universeobjects in the universe

increases as...increases as... mass increasesmass increases distance decreasesdistance decreases

GravityGravity

Who experiences more gravity - the astronaut Who experiences more gravity - the astronaut or the politician?or the politician?

less distance

more mass

Which exerts more gravity - Which exerts more gravity -

the Earth or the moon?the Earth or the moon?

GravityGravity WeightWeight

the force of gravity on an objectthe force of gravity on an object

MASSalways the same

(kg)

WEIGHTdepends on gravity

(N)

W = mgW: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)

GravityGravity Would you weigh more on Earth or Would you weigh more on Earth or

Jupiter?Jupiter?

greater gravitygreater gravity

greater weightgreater weight

greater massgreater mass

Jupiter because...Jupiter because...

GravityGravity Accel. due to gravity Accel. due to gravity (g)(g)

In the absence of air resistance, In the absence of air resistance, allall falling falling objects have the same acceleration!objects have the same acceleration!

On Earth:On Earth: g g = 9.8 m/s = 9.8 m/s22

mW

g

elephant

m

Wg

featherAnimation from “Multimedia Physics Studios.”

Newton’s First LawNewton’s First Law

Newton’s First Law of MotionNewton’s First Law of Motion An object at rest will remain at rest and An object at rest will remain at rest and

an object in an object in motionmotion will continue moving will continue moving at a at a constant velocityconstant velocity unless acted upon unless acted upon by a by a net forcenet force..

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

““Law of Inertia”Law of Inertia”

InertiaInertia tendency of an object to resist any change in its tendency of an object to resist any change in its

motionmotion increases as mass increasesincreases as mass increases

Newton’s Second LawNewton’s Second Law

Newton’s Second Law of MotionNewton’s Second Law of Motion The acceleration of an object is The acceleration of an object is directly proportionaldirectly proportional

to the net force acting on it and to the net force acting on it and inversely inversely proportionalproportional to its mass. to its mass.

F = ma

Newton’s Second LawNewton’s Second Law

F = maF: force (N)m: mass (kg)a: accel (m/s2)

1 N = 1 kg ·m/s2

am

F

a

Fm

CalculationsCalculations What force would be required to accelerate a 40 What force would be required to accelerate a 40

kg mass by 4 m/skg mass by 4 m/s22??

GIVEN:

F = ?

m = 40 kg

a = 4 m/s2

WORK:

F = ma

F = (40 kg)(4 m/s2)

F = 160 N

m

F

a

CalculationsCalculations A 4.0 kg shotput is thrown with 30 N of force. A 4.0 kg shotput is thrown with 30 N of force.

What is its acceleration?What is its acceleration?

GIVEN:

m = 4.0 kg

F = 30 N

a = ?

WORK:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2

m

F

a

CalculationsCalculations Mr. Keller weighs 745 N. What is his mass? Mr. Keller weighs 745 N. What is his mass?

GIVEN:

F(W) = 745 N

m = ?

a(g) = 9.8 m/s2

WORK:

m = F ÷ a

m = (745 N) ÷ (9.8 m/s2)

m = 76.0 kg

m

F

a

ConcepTestConcepTest

Is the following statement true or false?Is the following statement true or false? An astronaut has less mass on the moon An astronaut has less mass on the moon

since the moon exerts a weaker since the moon exerts a weaker gravitational force.gravitational force. False! Mass does not depend on gravity, weight False! Mass does not depend on gravity, weight

does. The astronaut has less does. The astronaut has less weightweight on the on the moon.moon.

Newton’s Third LawNewton’s Third Law Newton’s Third Law of MotionNewton’s Third Law of Motion

When one object exerts a force on a When one object exerts a force on a second object, the second object exerts second object, the second object exerts an equal but opposite force on the first.an equal but opposite force on the first.

Newton’s Third LawNewton’s Third Law Problem:Problem:

How can a horse How can a horse pull a cart if the cart pull a cart if the cart is pulling back on is pulling back on the horse with an equal but opposite force? the horse with an equal but opposite force?

NO!!!

Aren’t these “balanced forces” resulting in no Aren’t these “balanced forces” resulting in no acceleration?acceleration?

Newton’s Third LawNewton’s Third Law

forces are equal and opposite but act on forces are equal and opposite but act on differentdifferent objects objects

they are not “balanced forces”they are not “balanced forces” the movement of the horse depends on the the movement of the horse depends on the

forces acting forces acting on the horseon the horse

Explanation:Explanation:

Newton’s Third LawNewton’s Third Law

Action-Reaction PairsAction-Reaction Pairs

The hammer exerts a The hammer exerts a force on the nail to the force on the nail to the right.right.

The nail exerts an equal The nail exerts an equal but opposite force on but opposite force on the hammer to the left.the hammer to the left.

Newton’s Third LawNewton’s Third Law

Action-Reaction PairsAction-Reaction Pairs The rocket exerts a The rocket exerts a

downward force on the downward force on the exhaust gases.exhaust gases.

The gases exert an equal The gases exert an equal but opposite upward force but opposite upward force on the rocket.on the rocket.

FG

FR

Newton’s Third LawNewton’s Third Law

Action-Reaction PairsAction-Reaction Pairs Both objects accelerate.Both objects accelerate.

The amount of acceleration depends on the The amount of acceleration depends on the mass of the object.mass of the object.

a Fm

Small mass Small mass more acceleration more acceleration

Large mass Large mass less acceleration less acceleration

MomentumMomentum

MomentumMomentum quantity of motionquantity of motion

p = mvp: momentum (kg ·m/s)m: mass (kg)v: velocity (m/s)m

p

v

MomentumMomentum Find the momentum of a bumper car if it has a Find the momentum of a bumper car if it has a

total mass of 280 kg and a velocity of 3.2 m/s. total mass of 280 kg and a velocity of 3.2 m/s.

GIVEN:

p = ?

m = 280 kg

v = 3.2 m/s

WORK:

p = mv

p = (280 kg)(3.2 m/s)

p = 896 kg·m/s

m

p

v

MomentumMomentum The momentum of a second bumper car is 675 The momentum of a second bumper car is 675

kg·m/s. What is its velocity if its total mass is kg·m/s. What is its velocity if its total mass is 300 kg? 300 kg?

GIVEN:

p = 675 kg·m/s

m = 300 kg

v = ?

WORK:

v = p ÷ m

v = (675 kg·m/s)÷(300 kg)

v = 2.25 m/s

m

p

v

Conservation of MomentumConservation of Momentum

Law of Conservation of MomentumLaw of Conservation of Momentum The total momentum in a group of objects doesn’t The total momentum in a group of objects doesn’t

change unless outside forces act on the objects.change unless outside forces act on the objects.

pbefore = pafter

Conservation of MomentumConservation of Momentum Elastic CollisionElastic Collision

KE is conservedKE is conserved

Inelastic CollisionInelastic Collision KE is not conservedKE is not conserved

Conservation of MomentumConservation of Momentum A 5-kg cart traveling at 4.2 m/s strikes a A 5-kg cart traveling at 4.2 m/s strikes a

stationary 2-kg cart and they connect. Find their stationary 2-kg cart and they connect. Find their speed after the collision. speed after the collision.

BEFORECart 1:m = 5 kgv = 4.2 m/s

Cart 2 :m = 2 kgv = 0 m/s

AFTERCart 1 + 2:m = 7 kgv = ?

p = 21 kg·m/s

p = 0

pbefore = 21 kg·m/s pafter = 21 kg·m/s

m

p

vv = p ÷ mv = (21 kg·m/s) ÷ (7 kg)v = 3 m/s

Conservation of MomentumConservation of Momentum A 50-kg clown is shot out of a 250-kg cannon at a A 50-kg clown is shot out of a 250-kg cannon at a

speed of 20 m/s. What is the recoil speed of the speed of 20 m/s. What is the recoil speed of the cannon? cannon?

BEFOREClown:m = 50 kgv = 0 m/s

Cannon:m = 250 kgv = 0 m/s

AFTERClown:m = 50 kgv = 20 m/s

Cannon:m = 250 kgv = ? m/s

p = 0

p = 0

pbefore = 0

p = 1000 kg·m/s

pafter = 0

p = -1000 kg·m/s

Conservation of MomentumConservation of Momentum So…now we can solve for velocity. So…now we can solve for velocity.

GIVEN:

p = -1000 kg·m/s

m = 250 kg

v = ?

WORK:

v = p ÷ m

v = (-1000 kg·m/s)÷(250 kg)

v = - 4 m/s (4 m/s backwards)

m

p

v

Universal ForcesUniversal Forces

Electromagnetic ForcesElectromagnetic Forces – are associated – are associated with charged particles. The only force to with charged particles. The only force to attract and repel.attract and repel.

Universal ForcesUniversal Forces

Nuclear ForcesNuclear Forces – act within the nucleus – act within the nucleus of an atom to hold it together, strong and of an atom to hold it together, strong and weak.weak.

Universal ForcesUniversal Forces

Gravitational ForcesGravitational Forces – attractive forces – attractive forces that act between any two masses.that act between any two masses.

““Every object in the universe attracts Every object in the universe attracts every other object.” – Newton’s Law of every other object.” – Newton’s Law of Universal Gravitation.Universal Gravitation.

Centripetal ForceCentripetal Force

Centripetal force is a center-directed force Centripetal force is a center-directed force that continuously changes the direction of an that continuously changes the direction of an object to make it move in a circle. This object to make it move in a circle. This explains how the moon and satellites stay in explains how the moon and satellites stay in orbitorbit

““The Tide Is High…”The Tide Is High…” The gravitational pull from the moon The gravitational pull from the moon

produces two bulges in the Earth’s oceans. produces two bulges in the Earth’s oceans. One is on the side closest to the moon, and One is on the side closest to the moon, and the other is on the side farthest away from the other is on the side farthest away from the moon.the moon.