Download Study Notes Lesson 10 Newton`s Third Law of Motion

Physics Study Notes
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Lesson 10 Newton’s Third Law of Motion
Forces and Interactions
a. 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 that’s
drives the nail into the board and the nail exerts a force that halts the hammer.
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Newton’s Third Law
a. Newton’s 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
m 1 a1 = m2 a2
c. Action and reaction forces are acting on different objects, equal in strength and opposite in
direction. It doesn’t matter which one is called action and which one is called reaction.
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, the
water pushes you forward.
h. There cannot be action force without a reaction force.
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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 and
the reaction is the boulder exerting a force on Earth. The magnitudes of the two forces are the
same.
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Action and Reaction on Different Masses
a. 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 Newton’s 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. The
forces the cannon and cannonball exert on each other are equal and opposite. The cannon doesn’t
move as much as the cannonball since the cannon has much larger mass. But, cannon will still
recoil from 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 air
particles 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.
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Physics Study Notes
Lesson 10 Newton’s Third Law of Motion
e. Consider a soaring bird. The shape of its wings deflects air downward and the air pushes them up.
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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. Let’s say the apple and the orange are pulling the same string. View the orange in this diagram as a
“system”. 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 external to the system. Therefore the
action and reaction force don’t cancel out.
System
c. If we view the entire diagram as a system, then the forces are internal to the system and they cancel
out (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 the
two forces on the ball create a net force of zero.
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The Horse-Cart System
a. 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 cart’s acceleration.
c. In the horse system, the opposite reaction force by the cart restrains the horse. In order to accelerate
the 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 cart’s 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 the
cart 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.
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Action Equals Reaction
a. You cannot 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.)
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Action and Reaction Example Problems
a. Determine your weight on a bathroom scale placed in an elevator when the elevator is (a) stopped, (b) as it
accelerates upward, (c) as it travels at constant speed, (d) as it accelerates downward, and as it stops. What do
you expect to happen? Explain your reasoning in each case.
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Physics Study Notes
Lesson 10 Newton’s Third Law of Motion
b. 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.
c. Two cars are shown in the diagram. Car A is a front-wheel-drive car and car B is a rear-wheel-drive car. Please
identify all the forces acting on the car systems by drawing the free-body diagrams.
Car B
Car A
d. A 100 N pushing force at 30o angle is exerted on a 10 kg block against the wall. Identify and calculate all the
forces acting on the block by drawing the free-body diagram.
30o
e. Block A and block B are connected according to the following diagram. The readings of scale S1 and S2 are the
same 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?
S1
20 N
A
B
S2
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