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Physics Chapter 6 Newtons Third Law of Motion Study Notes

Action and Reaction

Mr. Lin 1

1 Forces and Interactionsa.An interaction is a mutual action between two things where each one exerts an equal force on

another.

b.Example: A hammer and a nail exert the same force on each other. The hammer exerts a force thatsdrives the nail into the board and the nail exerts a force that halts the hammer.

2 Newtons Third Lawa. Newtons third law: Whenever one objects exerts a force on a second object, the second object

exerts an equal and opposite force on the first object. (To every action there is always an equal

opposing reaction.)

b.Equation form:Action force = Reaction force

Faction = Freaction

m1 a1 = m2 a2

c.Action and reaction forces are equal in strength and opposite in direction.d.Force is not something an object has, like mass. Force is an interaction between two objects.e.Example: When you walk on the floor there is an interaction between you and the floor. When you

push against the floor, the floor pushes against you. The two forces are equal.f. Example: The tires of a car interact with the road. The tires push against the road and the road

pushes against the tires. The two forces are equal.

g.Example: When you go swimming you interact with the water. As you push the water backward, thewater pushes you forward.

h.There cannot be action force without a reaction force.3 Identifying Action and Reaction

a.An easy way to identify action and reactions forces is to simply say: Action: Object A exerts a force on Object B.

Reaction: Object B exerts a force on Object A.

b.Example: In the case of a falling boulder, the action is the Earth exerting a force on the boulder andthe reaction is the boulder exerting a force on Earth. The magnitudes of the two forces are thesame.

4 Action and Reaction on Different Massesa.When considering a boulder falling to Earth, you can say Earth is also falling to the boulder.

Although the forces are equal, the masses are very unequal. According to Newtons second law,

acceleration is directly proportional to the net force and inversely proportional to the mass.

Since Earths mass is so large, the acceleration is extremely small you cannot sense it.

b.When a cannonball is fired there is an interaction between the cannon and the cannonball. Theforces the cannon and cannonball exert on each other are equal and opposite. The cannon doesnt

move as much as the cannonball since the cannon has much larger mass. But, cannon will still

recoilfrom the cannonball it launches.

c.Comparison: Air escaping from an untied, blown-up balloon is the same as rocket propulsion. The balloon accelerates as the air comes out just as a rocket accelerates when it recoils from the

exhaust gases it releases from its engine.Rockets can be accelerated without the presence of an

atmosphere.

d.Consider a helicopter. The action force is caused the whirling blades that are shaped to force airparticles downward. The reaction force caused by the air forces the blades upward. This upward

reaction force is called the lift. If the lift is equal to the weight of the craft, the helicopter hovers

in midair. If the lift is greater than the weight of the craft, the helicopter accelerates upward.

e.Consider a soaring bird. The shape of its wings deflects air downward and the air pushes them up.

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Physics Chapter 6 Newtons Third Law of Motion Study Notes

Action and Reaction

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5 Do Action and Reaction Forces Cancel?a.Action and reaction forces are equal and opposite in direction. They do not cancel each other out

since they are acting on the different objects.

b.Lets say the apple and the orange are pulling the same string. View the orange in this diagram as asystem. The apple supplies an action force to the system and the system accelerates. There is a

reaction force the system exerting on the apple which is externalto the system. Therefore theaction and reaction force dont cancel out.

c.If we view the entire diagram as a system, then the forces are internal to the system and they cancelout (no acceleration.) However, if an external force was to act on this system, the action and

reaction forces would not cancel out ( it will accelerate.)

d.If two people kick a football with equal and opposite forces, the ball will not accelerate because thetwo forces on the ball create a net force of zero.

6 The Horse-Cart Systema.View the system in 3 different ways. The cart system, the horse system, and the horse-cart system.b.The cart system is only concerned with accelerating. The net force of the cart divided by the mass

of the cart is the carts acceleration.

c.In the horse system, the opposite reaction force by the cart restrains the horse. In order to acceleratethe horse must interact with the ground. In this interaction, the horse pushes the ground (action

force) and the ground pushes the horse (reaction force.) If the horse pushes the ground with a

greater force than the carts friction, then, it will accelerate. When the cart is at a decent speed, the

horse only needs to push the ground enough to offset the frictional force between the cart wheels

and the ground.

d.In the horse-cart system, the force the cart exerts on the horse and the force the horse exerts on thecart cancel out. In order for the entire system to accelerate, the horse-cart system must interact

with the ground. An outside force must act on the system in order for it to move.

7 Action Equals Reactiona.You can not exert a greater for on the wall than the wall can exert on you.b.You cannot exert a large force on objects such as a feather and a piece of paper because these

objects cannot exert a large force back. (Remember there cannot be a reaction force without an

action force.)

8 Action and Reaction Example Problemsa.In the horse-cart problem, if the horse and cart are moving forward, (a) identify all

the forces acting on the horse system by drawing the free-body diagram; (b) the

force that the horse pulling the cart is equal to the one that the cart pulling the

horse, explain why it is possible for horse and cart to move forward.

System

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Physics Chapter 6 Newtons Third Law of Motion Study Notes

Action and Reaction

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b. Two cars are shown in the diagram. Car A is a front-wheel-drive car and car B is a rear-wheel-drive car. Pleaseidentify all the forces acting on the car systems by drawing the free-body diagrams.

c.A 100 N pushing force at 30o angle is exerted on a 10 kg block against the wall. Identify and calculate all theforces acting on the block by drawing the free-body diagram.

d.Block A and block B are connected according to the following diagram. The readings of scale S1 and S2 are thesame and the reading of the middle one is 20 N. If the system is frictionless and block A is 10 kg, what is the

mass of block B?

Homework:

1. Reading: Conceptual Physics, chapter 6.2. Conceptual Physics, p. 83-85, Question # 3, 6, 8, 12, 13, 14, 16, 17, 33, 35 and 36.

30o

S1

S2

20 N

A

B

Car A Car B