newton’s 3 laws of motion unit 2c. forces cause changes in motion. newton’s 1 st law (inertia )

Newton’sNewton’s3 Laws of Motion3 Laws of Motion

Unit 2C

Forces cause changes in motion.

Newton’s 1Newton’s 1stst Law Law(Inertia(Inertia))

A ball is at rest in the middle of a flat field.

No net force acts on it.

If it began to move, you’d look for what caused it to move.

We don’t believe that changes in motion occur without cause.

Force causes changes in motion

Natural motion on Earth was thought to be either straight up or straight down.

• Objects seek their natural resting places.

• Heavy things fall and very light things rise.

• Circular motion was natural for the heavens.

• These motions were considered natural–not caused by forces.

4.1 Aristotle on Motion

2 types of motion: natural and violent

Violent motion was imposed (forced) motion. • Caused by forces that pushed or pulled.

• Cause comes from outside the object.

• Objects in their natural resting placescould not move by themselves.


2 types of motion: natural and violent

For nearly 2000 years it was thought that a force was responsible for an object moving “against its nature.”

• Objects stayed at rest unless being pushed or pulled or moving to their natural resting place.

• Before the 1500s it was obvious that Earth must be in its natural resting place.

• A force largeenough to move itwas unthinkable.

• Earth did not move.


This idea was extremely controversial at the time. People believed Earth was at the center of the universe.He worked on his ideas in secret.

The first copy of his work reached

him the day he died, May 24, 1543.

4.2 Copernicus and the Moving Earth

Copernicus proposed that Earth moved around the sun.

Galileo, the foremost scientist of late-Renaissance Italy, was outspoken in his support of Copernicus.

One of Galileo’s great contributions to physics was demolishing the notion that a force is necessary to keep an object moving.

Force:a push or a pull

4.3 Galileo on Motion

Friction:

force that acts between materials that touch as they move past each other.

• caused by the irregularities (bumpies) in the surfaces of objects that are touching.

• With no friction, a moving object would need no force at all to remain in motion.


block

table


Galileo tested his idea by rolling balls along plane surfaces tilted at different angles.

withgravity



againstgravity

withgravity



againstgravity

withgravity

constantvelocity

a. The ball rolling down the incline rolls up the opposite incline and reaches its initial height.



b. The ball rolls a greater distance to reach its initial height.



b. The ball rolls a greater distance to reach its initial height.

c. If there is no friction, the ball will never stop.


Galileo stated that every object resists changes to its state of motion and tends to keep moving.

Inertia: property of an object –- to resist changes in motion- if in motion, to continue in motion- if at rest, to stay at rest


c. If there is no friction, the ball will never stop.

According to Galileo, when is a force needed to keep an object moving? NEVER!!

One arguments against the moving Earth theory of Copernicus was:

• the bird sees a worm, drops down vertically, and catches it.

• this would not be possible if Earth moved

4.9 The Moving Earth Again

Objects Move With Earth

You can refute this argument using the idea of inertia.

Earth moves at 30 km/s, butso does the tree, the worm below,and even the air in between.


Flip a coin in an airplane, and it behaves as if the plane were at rest.

The coin keeps up with you instead of moving back as you move forward.

Inertia in action!


Objects Move With Vehicles

Objects in circular (orbital) motion will continue in a line tangent to the motion without a force changing their direction.

Inertia for circular paths:

BD

C

A

ball motion pathE

same path

line tangent to path

line directly away from

center

spiral closer in

spiral further out

1. According to Aristotle, Earth was at the center of the universe. The first European to effectively challenge that notion was…

A. Copernicus

B. Galileo

C.Newton

D.Einstein

Quick Quiz!

2. Galileo’s conclusions about motion helped advance science because they were based on…

A. experiments rather than philosophical discussions.

B. philosophical discussions rather than experiments.

C.nonmathematical thinking.

D.Aristotle’s theories of motion.

Quick Quiz.

3. If gravity between the sun and Earth suddenly vanished, Earth would continue moving in which path?

Quick Quiz.

BD

C

A

Earth’s normal

orbit pathEsame orbit

line tangent to orbit

line directly away from sun

spiral closer to sun

spiral further

from sun

4. The Earth moves at 30 km/s relative to the sun. But when you jump upward in front of a wall, the wall doesn’t slam into you at 30 km/s because…

A. the sun’s influence on you is negligible.

B. the air in the room is also moving.

C.both you and the wall are moving at the same speed, before, during, and after your jump.

D. the inertia of you and the wall is negligible compared with that of the sun.

Quick Quiz.

NEWTON’S 3 LAWS of NEWTON’S 3 LAWS of MOTION – Video MOTION – Video (4:51)(4:51)

NEWTON’S 3 LAWS of NEWTON’S 3 LAWS of MOTION – Video MOTION – Video (11:39)(11:39)

http://www.youtube.com/watch?v=NWE_aGqfUDs&feature=watch-vrec&safety_mode=true&persist_safety_mode=1&safe=active

http://www.youtube.com/watch?v=NYVMlmL0BPQ

Newton’s 1st Law: Inertia

4.4 Newton’s 1st Law: Inertia

Simply put, things tend to keep on doing what they’re already doing.

• Objects at rest will remain at rest. (unless acted on by an unbalanced force)

• Objects in motion will continue moving in a straight line at a constant speed. (unless acted on by an unbalanced force)


• Objects at rest tend will remain at rest, unless acted on by an unbalanced force.

If you slide the puck along the surface of a the street, it soon comes to rest, but if you slide it along an air table where friction is practically absent, it slides with no apparent loss in speed.


• Objects in motion will continue moving in a straight line at a constant speed,unless acted on by an unbalanced force.

• Objects continue to move by themselves. • Forces are needed to overcome any friction that may be

present and to set objects in motion initially.

• Once the object is moving with balanced forces (or no force), it will move in a straight line at constant speed forever.


• objects moving in outer space will move forever due to inertia.

frictionpush


• Objects at rest tend; will remain at rest, unless acted on by an unbalanced force.


• Objects in motion will continue moving in a straight line at a constant speed,unless acted on by an unbalanced force.

The amount of inertia an object has depends on its mass—which is roughly the amount of material present in the object.

Mass is a measure of the inertia of an object.

4.5 Mass—A Measure of Inertia

You can tell how much matter is in a can when you kick it.

Which can “resists” your kick more?

empty can

or

soda-filled can

Why?more mass, more inertia

The pillow has a larger size The pillow has a larger size (volume) but a smaller mass (volume) but a smaller mass than the battery. than the battery.


Which has more mass, a pillow or a car battery?

Mass Is Not Volume

• Volume is a measure of space (cm3, mL, L)

• Mass is a measure of inertia (kg, kilograms)

(clearly a car battery is more difficult to set into motion)

The battery has more inertia The battery has more inertia because it has more massbecause it has more mass.

Mass Is Not Weight


Mass is often confused with weight.• Mass is the

amount of matterin an object.

• Weight is theforce of gravity actingon an object.

Mass Is Inertia• Mass is a measure of inertia

(resistance to changes its motion)

Mass Is Inertia

A stone’s mass is the same whether the stone is on Earth, on the moon, or in outer space.

but…

A stone’s weight would bevery different on Earth, on the moon, or in outer space.


The stone’s inertia, or mass, is a property of the stone and not its location.

1 Kilogram = 9.8 Newtons


The unit of mass is the kilogram (kg)

The unit of force is the newton (N)

W = (1 kg)(9.8 m/s2)

W = 9.8 N

9.8 N = 1 kg

2.2 lb. = 1 kg

W = mg (weight)

TRUE or FALSEAn astronaut has less mass on the moon since the moon exerts a weaker gravitational force.

1 Kilogram = 10 Newtons


The unit of mass is the kilogram (kg)

The unit of force is the newton (N)

W = mg (weight)

1. Does a 2 kg bunch of bananas have twice as much mass as a 1 kg loaf of bread?

…twice as much inertia?

…twice as much weight?

…twice as much volume?

Quick Quiz!

yes

yes

yes

no

2. To say that 1 kg of matter weighs 9.8 N is to say that 1 kg of matter…

A. will weigh 9.8N everywhere.

B. has ten times more mass than 9.8 kg of matter.

C.has ten times more inertia than 9.8 kg of matter.

D. is attracted to Earth with 9.8 N of force.

Quick Quiz.

3. What is the mass of a book that weighs 35 N?

A. 350.4 kg

B. 77.1 kg

C. 16.0 kg

D. 3.57 kg

Quick Quiz.

W = mg

35 = m(9.8)

NEWTON’S LAW #1Video 6:32

NEWTON’S LAW #1 Inertia

Video 6:33

http://www.youtube.com/watch?v=BlFGN2zlDYc

http://www.youtube.com/watch?v=Q0Wz5P0JdeU&safety_mode=true&persist_safety_mode=1&safe=active

http://www.youtube.com/watch?v=Q0Wz5P0JdeU&safety_mode=true&persist_safety_mode=1&safe=active

FREE BODY FREE BODY DIAGRAMSDIAGRAMS

Force:

push or a pull

Net Force: Fnet

sum of all forcesacting on an object

4.6 Net Force

What’s the magnitude and direction of the Net Force?

4.6 Net Force

Force:

push or a pull

Net Force: Fnet



4.6 Net Force

Force:

push or a pull

Net Force (Fnet):



4.6 Net Force

upward force

=downward force

Fnet = 0

4.6 Net Force

Tension (T):stretching force in a string, rope, or spring.

The bag of sugar is attracted to Earth with a gravitational force of 2.2 pounds (or 9.8 newtons).

4.6 Net Force

Tension (T):stretching force in a string, rope, or spring.

The two forces on the bag are equal and opposite. The net force on the bag is zero, so it remains at rest.

T

W

Tup = Wdown

Tup = Wdown

Mechanical Equilibrium:

no changes in motion (Fnet = 0)

(no forces or balanced forces)

4.7 Equilibrium

( means “sum of”)F = 0

Equilibrium:



4.7 Equilibrium

Equilibrium:



4.7 Equilibrium

Wm2Wm1

Wb

Equilibrium:



4.7 Equilibrium

F = 0no changes in motion

Wm2Wm1

Wb

T1 T

2

T1 + T

2 – Wm1

– Wm2 – Wb = 0

T1 + T

2 = Wm1

+ Wm2 + Wb

4.7 Equilibrium

W = mg

What forces act on a book at rest on a table?

• W (down due to gravity)

• Fnet must be 0 b/c no change in motion

• There must be an upward forceopposite to the force of gravity.

4.7 Equilibrium

W = mg

support force:

upward force that balances an object’s weight on a surface, or normal force (n)

What forces act on a book at rest on a table?

• W (down due to gravity)

• Fnet must be 0 b/c no change in motion

• There must be an upward forceopposite to the force of gravity n = –W

n – W = 0, or F = 0

4.7 Equilibrium

think!What is the net force on abathroom scale when a110 lb. person stands on it?

4.7 Equilibrium

Remember:

n = –W

think!What is the net force on abathroom scale when a110 lb. person stands on it?

Zerothe scale is at rest and itreads the support force,not the net force.

Remember:

An object at rest is in equilibrium b/c F = 0.

Dynamic Equilibrium:

moving at constant speed in a straight-line

F = 0 (same as not moving)

4.7 Dynamic Equilibrium

Dynamic Equilibrium:

moving at constant speed in a straight-line

F = 0 (same as not moving)

Once in motion, if there is no net force to change the state of motion, it is in equilibrium.

4.7 Dynamic Equilibrium

FfrictionFpush

FREE BODY FREE BODY DIAGRAMSDIAGRAMS

Quick Quiz!

1. When you hold a rock in your hand at rest, the forces on the rock…

A. are only to gravity.

B. are only the upward push of your hand.

C. cancel to zero.

D. don’t act unless the rock is dropped.

2. Burl and Paul have combined weights of 1300 N. The tensions in the ropes that support the scaffold add to 1700 N. The weight of the scaffold itself must be…

A. 400 N

B. 500 N

C. 600 N

D. 3000 N

Quick Quiz.

Wboard = ? WPWB

T1

T2

3. Harry gives his little sister a piggyback ride. Harry weighs 400 N and his little sister weighs 200 N. The support force supplied by the floor must be…

A. 200 N

B. 400 N

C. 600 N

D. more than 600 N

Quick Quiz.

(draw a free-body diagram)(a picture showing forces)

4. When a desk is horizontally pushed across a floor at a constant speed in a straight-line direction, the amount of friction acting on the desk is…

A. less than the pushing force.

B. equal to the pushing force.

C. greater than the pushing force.

Quick Quiz.

Part IIPart II

Newton’s 2Newton’s 2ndnd Law ( Law (F F = = mama))

An object accelerates when a net force acts on it.

Recall the definition of acceleration:

Force is thecause of

acceleration.

For Example:

When a hockey puck is at rest, the net force on it (gravity and the support force) is balanced, so the puck is in equilibrium.

Hit the puck (apply an unbalanced force to it) and the puck experiences a change in motion—it accelerates.

5.1 Force Causes Acceleration

Remember: F = Fnet

• acceleration depends on the net force

• for an object to accelerate, you must apply a net force (unbalanced).

• Force is the cause of acceleration.

5.1 Force Causes Acceleration

Push on an empty shopping cart.

Then push equally hard on a heavily loaded shopping cart.

The loaded shopping cart will accelerate much less than the empty cart.

- acceleration depends on the mass

•5.2 Mass Resists Acceleration

more Fnet , more a

more mass , less a

•5.2 Mass Resists Acceleration

In Summary:(causes a)

(resists a)

•5.3 Newton’s Second Law

Newton’s 2nd Law describes the relationship of an object's mass, acceleration, and the net force on an object.

Newton’s 2nd Law: F = ma

a = Fm

F = ma

• If the net force acting on an object doubles, its acceleration is doubled.

• If the mass is doubled, then acceleration will be halved.

• If both the net force and the mass are doubled, the acceleration will be unchanged.


a = Fm

F = ma a = Fm

a = F

m

a = Fm

Example 1:

If a car can accelerate at 2 m/s2, what acceleration can it attain if it is towing another car of equal mass?

The same force on twice the mass produces half the acceleration, or 1 m/s2.

5.3 Newton’s Second Law

a = Fm

2 = Fm ? =

F2m 1 =

F2m

Example 2:

A car has a mass of 1000 kg. What is the acceleration produced by a force of 2000 N?


a = Fm a =

Fm =

20001000 = 2 m/s2 m/s22

Example 2:

A car has a mass of 1000 kg. What is the acceleration produced by a force of 2000 N?


a = Fm a =

Fm =

20001000 = 2 m/s2 m/s22

If the force is 4000 N, what is the acceleration?

a = Fm =

40001000 = 4 m/s4 m/s22double F

double a

Example 3:

How much force, or thrust, must a 30,000 kg jet plane develop to achieve an acceleration of 1.5 m/s2?


a = Fm

F = ma

F = ma= (30,000)(1.5)

45,000 N45,000 N=

1. An object will accelerate when…

A. ∑F = 0.

B. more than one force acts on it.

C. it is pushed or pulled with a net force.

D. its mass increases.

•Quick Quiz!

2. When a net force acts on an object, its acceleration depends on the object’s…

A. initial speed.

B. mass.

C. volume.

D. weight.

•Quick Quiz.

NEWTON’S LAW #2VIDEO

http://www.youtube.com/watch?v=-KxbIIw8hlc&feature=relmfu

3. A cart is pushed and undergoes a certain acceleration. If it were pushed with twice the net force and its mass was increased by four times as much, then its acceleration would be…

A. one fourth as much.

B. half as much.

C. twice as much.

D. the same.

•Quick Quiz.

?a = 2F4m

½a = 2F4m

Friction is a force and affects motion:

• Acts on materials in contact with each other.

• It always acts in an opposite direction to relative motion.

• Mainly due to irregularities (bumpies) in the two surfaces.

5.4 Friction

A concrete road divider has a better design than a steel road divider for slowing an out-of-control, sideswiping car.

5.4 Friction

Rubber against concrete produces more friction than steel against steel.

Air resistance is the friction acting on something moving through air.

5.4 Friction

free-body diagram:

shows all the forces acting on an object.

The direction of the force of friction always opposes the direction of motion.

Push the crate to the right and friction acts toward the left.

If Ffrict. = Fpush then F = 0 even when moving.

5.4 Friction

The direction of the force of friction always opposes the direction of motion.

a. Push the crate to the right and friction acts toward the left.

b. The sack falls down and air friction acts up.

5.4 Friction

think!

Two forces act on a book resting on a table: its weight (W) and the support force (n) from the table.

Does a force of friction act as well?

No.Not unless the book is sliding

across the table. Friction forces

occur only when an object is in motion

5.4 Friction

Newton’s Second Law

Honors Physics

N.S.L."The acceleration of an object is directly

proportional to the NET FORCE AND inversely proportional to the mass."

ma

Fa NET

1

Acceleration is directly proportional to the NET Force.DIRECTLY = They do the same thing. If the force increases, the acceleration increases. If the force decreases, the acceleration decreases.

Acceleration is inversely proportional to the mass.INVERSELY = They do the opposite.If the mass decreases, the acceleration will increase. If the mass increases, the acceleration will decrease.

N.S.L.

N.S.L. works based on these direct and inverse relationships. As 2 of the variable change, ONE of them must remain constant.

If the force is constant, the acceleration and mass change as shown above.

F(net)=ma2F=m(2a)3F=m(3a)

If we add a second dog pulling with 100N just like the first dog, we could pull the sled with twice the acceleration, provided the mass of the sled was constant.

Putting it all together

NETFam

a1

0

Force Total

NET

NET

NETNET

F

FF

maFm

Fa

10 N 3 N

Magnitude of FNET=

Direction =

Acceleration =

7 N

RIGHT

10 kg

0.70 m/s/s

Newton’s 2nd Law Tips1. Draw a free body diagram

2. Break vectors into components

3. Find the NET force by adding and subtracting forces that are on the same axis as the acceleration.

4. Set net force equal to “ma” this is called writing an EQUATION OF MOTION.

NOTE: To avoid negative numbers, always subtract smaller forces from the larger.

ExampleA 50 N applied force drags an 8.16 kg log to the right

across a horizontal surface. What is the acceleration of the log if the force of friction is 40.0 N?

50 N40 N

mg

Fn

a

a

a

a

maFF

maF

fa

NET

16.810

16.84050

1.23 m/s/s

FFf f = Force of Friction= Force of Friction

FFn n = Normal Force= Normal Force

FFa a = Applied Force= Applied Force

mgmg = Force of Gravity = Force of Gravity

ExampleAn elevator with a mass of 2000 kg rises with an

acceleration of 1.0 m/s/s. What is the tension in the supporting cable?

mg

T

T

T

mgmaT

mamgT

maFNET

)8.9)(2000()1)(2000(

Equation of Motion

21,600 N

ExampleA sled is being accelerated to the right at a rate of 1.5 m/s/s by a rope at a 33 degree angle above the horiz. Calculate the Frictional Force if the mass of the sled is 66 kg and the tension in the rope is 150 N.

mg

FN T

Ff

Tcos

Tsin

f

f

f

f

NET

F

F

FmaT

FT

F

)5.1)(66(33cos150

cos

macos

ma

x

x

26.8 N

In Galileo’s famous demonstration, a 10-kg cannonball and a 1-kg stone strike the ground at practically the same time.

This experiment demolished the Aristotelian idea that an object that weighs ten times as much as another should fall ten times faster than the lighter object.

•5.6 Free Fall

5.6 Free Fall

Recall that mass (a quantity of matter) and weight (the force due to gravity) are related.

• A 10-kg cannonball experiences 10 times as much gravitational force (weight) as a 1-kg stone.

• Newton’s 2nd Law tells us to consider the mass as well.

5.6 Free Fall

W = mg a = Fm

F = ma

The weight of a 1-kg stone is 10 N at Earth’s surface. Using Newton’s second law, the acceleration of the stone is

The weight of a 10-kg cannonball is 100 N at Earth’s surface and the acceleration of the cannonball is

5.6 Free Fall

The ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the1-kg stone.

5.6 Free Fall

10 times as much force acting on 10 times as much mass produces the same acceleration.

Speed and Area

You experience the force due to air resistance when you stick your hand out of the window of a moving car.

• If the car moves faster, the force on your hand increases.

• If instead of just your hand, you hold your physics book out the window with the large side facing forward, the air resistance force is much larger than on your hand at the same speed.

5.7 Falling and Air Resistance

Terminal Velocity:

speed at which the acceleration of a falling object is zero because friction balances the weight.


more speed, more air resistance

more area, more air resistance

Sky divers reach terminal velocity when air resistance equals weight.


150 to 200 km/h

more speed?less speed?more weight

more area

Parachute!!

This stroboscopic photo shows a golf ball and a foam ball falling in air.

The heavier golf ball is more effective in overcoming air resistance, so its acceleration is greater.


At low speeds, air resistance is often ignored, but at high speeds, it can make quite a difference.

(your car vs. a race car design)

think!If a heavy person and a light person open their parachutes together at the same altitude and each wears the same size parachute, who will reach the ground first?

The heavy person will reach the ground first.

Like a feather, the light person reaches terminal speed sooner, while the heavy person continues to accelerate until a greater terminal speed is reached.


2. The reason a 20-kg rock falls no faster than a 10-kg rock in free fall is that…

A. air resistance is negligible.

B. the force of gravity on both is the same.

C. their speeds are the same.

D. the force/mass ratio is the same.

•Quick Quiz.

3. Kevin and Suzanne go sky diving. Kevin is heavier than Suzanne, but both use the same size parachute. Kevin has a greater terminal speed compared with Suzanne because…

A. he has to fall faster for air resistance to match his weight.

B. gravity acts on him more.

C. he has greater air resistance.

D. he has weaker terminal velocity.

Quick Quiz.

Part III:

Newton’s 3rd Law (action/reaction)

For every force, there is an equal and For every force, there is an equal and opposite force.opposite force.

NEWTON’S

3rd LAW

(Bill Nye)

http://www.youtube.com/watch?v=NRKmJgIokxg&feature=related&safety_mode=true&persist_safety_mode=1&safe=active

A force is a push or a pull.

When you push on the wall, the wall pushes on you.

6.1 Forces and Interactions

6.1 Forces and Interactions

A hammer exerts a force on the nail and drives it into a board.

There must also be a force exerted on the hammer to stop it.

There is ALWAYSa pair of forces.(never just one force)

In this case,one acting on the nail and theother acting on the hammer.

Newton’s 3rd Law: action/reaction forces

For every action force on an object, there is an equal and opposing reaction force on the OTHER object.

One force is called the action force.

The other force is the reaction force.

Neither force exists without the other.

They’re equal in strengthand opposite in direction.

Do NOT cancel out(they’re on different objects)

6.2 Newton’s 3rd Law

Forces always occur in PAIRS.

You push against the floor, and the floor simultaneously pushes against you.

The tires of a car push against the road, and the road simultaneously pushes back on the tires.

When swimming, you push the water backward, and…the water pushes you forward.


In many cases, the interactions depend on friction as one of the forces in the pair.


ON ICEON ICE friction is minimal and you may not be able to exert an action force against the ice.

Without the action force there cannot be a reaction force, and there is no resulting forward motion.

There is a simple process for treating action and reaction forces:

• First identify the interaction. Let’s say one object, A, interacts with another object, B.

• State the action and reaction forces in the form:

Action: A exerts a force on B.Reaction: B exerts an equal force on A.

6.3 Action and Reaction Pairs

Sometimes the pair of action and reaction forces in an interaction is not obvious.

In the case of a falling boulder, what are the action and reaction forces?

If we call the action Earthexerting a force on the boulder,then the reaction is…the boulder exerting a force on Earth.


If action is A exerts force on B, then reaction is B exerts equal force on A.


think!We know that Earth pulls on the moon.

Does the moon also pull on Earth?

Which pull is stronger?

Which distance is greater—between New York and L.A., or between L.A. and New York?

The distances either way are the same.

It’s the same with force pairs.

Both Earth and moon pull on each other with equal and opposite forces.


yes

equal force, but opposite direction

Earth is pulled up by the boulder with just as much force as the boulder is pulled down by Earth.

6.4 Action and Reaction on Different Masses

The forces are equal in strength and opposite in direction.

Although the pair of forces between the boulder and Earth is the same, the masses are NOT equal.

same F, but...more m, less a.

Because Earth has a huge mass, we don’t sense its super tiny acceleration.

The force a cannon exerts on a cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon, but…the cannonball moves much faster than the cannon.

Newton’s 2nd Law shows that we must also consider the masses.


a = Fm

F = ma

The cannonball has more acceleration than the cannon because it has less mass.


a = Fm

F = ma


Cannon recoil from the cannonball launch can help us understand rocket propulsion.

The air accelerates backward from the force of the balloon on it.

The balloon accelerates forward from the force of air on it.


A rocket accelerates as it continually recoils from the exhaust gases ejected from its engine.

Each molecule of exhaust gas acts like a tiny molecular cannonball shot downward from the rocket.

Cannon recoil from the cannonball launch can help us understand rocket propulsion.

Lift

Using Newton’s 3rd Law, we can understand how a helicopter gets its lifting force.

The whirling blades force airparticles downward (action).

The air forces the bladesupward (reaction).

This upward reaction force is called lift.

When lift equals the weight,the helicopter hovers.When lift is greater, it climbs upward.


1. Every interaction requires at least…

A. a single force.

B. a pair of forces.

C. an action force.

D. a reaction force.

Quick Quiz!

2. Whenever one object exerts a force on a second object, the second object exerts a force on the first object that is…

A. opposite in direction and equal in magnitude.

B. in the same direction and equal in magnitude.

C. opposite in direction and greater in magnitude.

D. in the same direction and weaker in magnitude.

Quick Quiz.

3. The force that directly propels a car along a road is that provided by the…

A. engine.

B. fuel.

C. tires.

D. road.

•Quick Quiz.

4. When you jump vertically upward, what does the Earth do?

A. move downward

B. also move upward with you

C. remain stationary

D.move sideways a bit

Quick Quiz.

Since action and reaction forces are equal and opposite, why don’t they cancel to zero?

To answer this question, we must consider the system involved.

6.5 Defining Systems


system

The system (orange) accelerates from force.

forceON orange

The action force ON orange (from apple) is not cancelled by the reaction force ON apple. The orange still accelerates.

reaction force ON apple

a. Action and Reaction forces cancel b/c…

IN systemNOT

ON system


system

a. Action and Reaction forces cancel b/c…

IN systemNOT

ON system

b. apple pushes on floor, so floor pushes on apple

The orange-apple system accelerates.


system

force ONorange-apple system

Inside a baseball, trillions of atomic/molecular forces hold the ball together but do not accelerate the ball.

They are part of action-reaction pairs within the ball, but they combine to zero.

If the action-reaction forces are internal to the system, then they canceland the system doesnot accelerate.

An external force,such as a swinging batis needed toaccelerate the ball.


think!Suppose a friend who hears about Newton’s 3rd Law says that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to your kicking force. The net force would be zero, so no matter how hard you kick, the ball won’t move! What do you say to your friend?

If you kick a football, it will accelerate b/c no other force has been applied to the ball.

(the ball applies a reaction force to your foot, not a cancelling force on the ball)


6.6 The Horse-Cart Problem

IN system

ON systemON systemON ground

Will the horse’s pull on the cart be canceled by the opposite and equal pull by the cart on the horse, thus making acceleration impossible?

3 pairs of forces: P/P F/F f/f

IN system: P forward – P backward (cancels)

ON system: F forward – f backward



•IN system

ON system: F forward – f backward

F = F – f = Fnet



IN system

a = Fnet

mcart+horse

If you hit the wall, it will hit you equally hard.

6.7 Action Equals Reaction

You cannot touch without being touched.

You can only be apply a force that can be applied back to you.

How hard can you hit a piece of paper in the air?

1. A system undergoes acceleration only when acted on by a(n)…

A. net force.

B. pair of forces.

C.action and reaction forces.

D. internal interactions.

Quick Quiz!

2. If a net force acts on a horse while it is pulling a wagon, the horse…

A. accelerates.

B. is restrained.

C. is pulled backward by an equal and opposite net force.

D.cannot move.

Quick Quiz.

3. At a pizza shop, the cook throws the pizza dough in the air. The amount of force the cook exerts on the dough depends on the…

A. mass of the dough.

B. strength of the cook.

C.weight of the dough.

D.height of the cook.

Quick Quiz.

•The cook can only exert a force on the dough that is equal to what the dough can exert back on him.

Newton’s 3rd Law and Law of Gravitation

Honors Physics

Newton’s Third Law“For every action there is an EQUAL and

OPPOSITE reaction. This law focuses on action/reaction pairs (forces) They NEVER cancel out

All you do is SWITCH the wording!•PERSON on WALL•WALL on PERSON

N.T.LThis figure shows the force during a collision between a truck and a train. You can clearly see the forces are EQUAL and OPPOSITE. To help you understand the law better, look at this situation from the point of view of Newton’s Second Law.

TrainTrainTruckTruck

TrainTruck

aMAm

FF

There is a balance between the mass and acceleration. One object usually has a LARGE MASS and a SMALL ACCELERATION, while the other has a SMALL MASS (comparatively) and a LARGE ACCELERATION.

N.T.L ExamplesAction: HAMMER HITS NAILReaction: NAIL HITS HAMMER

Action: Earth pulls on YOUReaction: YOU pull on the earth

newton’s 3 laws of motion unit 2c. forces cause changes in motion. newton’s 1 st law (inertia )

Documents

force increases

force decreases

acceleration increases

acceleration decreases

force of frictionfn

frictional force

mass increases

gravitational force