newton 3rd law of motion

7/26/2019 Newton 3rd Law of Motion

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Newtons Third Law

The Third Law tells us that:

To every action, there is an equal and opposite reaction.

What this tells us in physics is that every push or pull produces not one, but two forces. In any exertion

of force, there will always be two objects: the object exerting the force and the object on which the

force is exerted. Newtons Third Law tells us that when object A exerts a force F on object B, object B

will exert a forceFon objectA. When you push a box forward, you also feel the box pushing back on

your hand. If Newtons Third Law did not exist, your hand would feel nothing as it pushed on the box,

because there would be no reaction force acting on it.

The statement means that in every interaction, there is a pair of forces acting on the two interacting

objects. The size of the forces on the first object equals the size of the force on the second object. The

direction of the force on the first object is opposite to the direction of the force on the second object.Forces always come in pairs - equal and opposite action-reaction force pairs.

Newtons Third Law tells us that the force that the skater exerts on the wall, , is exactly equal in

magnitude and opposite in direction to the force that the wall exerts on the skater, . The harder

the skater pushes on the wall, the harder the wall will push back, sending the skater sliding backward.

Examples of Newtons Third Law at work:

You push down with your hand on a desk, and the desk pushes upward with a force equal in

magnitude to your push.

A brick is in free fall. The brick pulls the Earth upward with the same force that the Earth pulls

the brick downward.

When you walk, your feet push the Earth backward. In response, the Earth pushes your feet

forward, which is the force that moves you on your way.

The second example may seem odd: the Earth doesnt move upward when you drop a brick. Butrecall

Newtons Second Law: the acceleration of an object is inversely proportional to its mass (a= F/m). The

Earth is about 1024 times as massive as a brick, so the bricks downward acceleration of 9.8 m/s2 is

about 1024 times as great as the Earths upward acceleration. The brick exerts a force on the Earth, but

the effect of that force is insignificant.


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A force is a push or a pull upon an object that results from its interaction with another object. Forces

result from interactions!

According to Newton, whenever objects A and B interact with each other, they exert forces upon each

other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts

an upward force on your body. There are two forces resulting from this interaction - a force on the chair

and a force on your body. These two forces are called actionand reactionforces and are the subject ofNewton's third law of motion.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through

the water. A fish uses its fins to push water backwards. But a push on the water will only serve to

accelerate the water. Since forces result from mutual interactions, the water must also be pushing the

fish forwards, propelling the fish through the water. The size of the force on the water equals the size of

the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the

force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction)

reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird

push air downwards. Since forces result from mutual interactions, the air must also be

pushing the bird upwards. The size of the force on the air equals the size of the force on

the bird; the direction of the force on the air (downwards) is opposite the direction of the

force on the bird (upwards). For every action, there is an equal (in size) and opposite (in

direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Check Your Understanding

1.

While driving down the road, a firefly strikes the windshield of a bus

and makes a quite obvious mess in front of the face of the driver. This

is a clear case of Newton's third law of motion. The firefly hit the bus

and the bus hits the firefly. Which of the two forces is greater: the

force on the firefly or the force on the bus?

2.

For years, space travel was believed to be impossible because there was nothing that rockets

could push off of in space in order to provide the propulsion necessary to accelerate. This

inability of a rocket to provide propulsion is because ... ...............

a.

... space is void of air so the rockets have nothing to push off of.

b.

... gravity is absent in space.

c.

... space is void of air and so there is no air resistance in space.

d.

... nonsense! Rockets do accelerate in space and have been able to do so for a long time.

3.

Many people are familiar with the fact that a rifle recoils when fired. This recoilis the result of action-reaction force pairs. A gunpowder explosion creates hot

gases that expand outward allowing the rifle to push forward on the bullet.

Consistent with Newton's third law of motion, the bullet pushes backwards

upon the rifle. The acceleration of the recoiling rifle is ...

a. greater than the acceleration of the bullet.

b. smaller than the acceleration of the bullet.

c. the same size as the acceleration of the bullet.


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4. In the top picture (below), Kent Budgett is pulling upon a rope that is attached to a wall.In the bottom picture, the Kent is pulling upon a rope that is attached to an elephant. In

each case, the force scale reads 500 Newton. Kent is pulling .................

a. with more force when the rope is attached to the wall.

b. with more force when the rope is attached to the elephant.

c. the same force in each case.

ANSWERS:

1)

Each force is the same size. For every action, there is an equal ... (equal!). The fact that the

firefly splatters only means that with its smaller mass, it is less able to withstand the larger

acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a

tendency to be splatterable. Windshields don't have guts. There you have it.

2)

It is a common misconception that rockets are unable to accelerate in space. The fact is that

rockets do accelerate. There is indeed nothing for rockets to push off of in space - at least

nothing which is external to the rocket. But that's no problem for rockets. Rockets are able to

accelerate due to the fact that they burn fuel and push the exhaust gases in a direction oppositethe direction which they wish to accelerate.

3)

The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and

mass. The bullet has a greater acceleration due to the fact that it has a smaller mass.

Remember: acceleration and mass are inversely proportional.

4)

Kent is pulling with 500 N of force in each case. The rope transmits the force from Kent to the

wall (or to the elephant) and vice versa. Since the force of Kent pulling on the wall and the wall

pulling on Kent are action-reaction force pairs, they must have equal magnitudes. Inanimate

objects such as walls can push and pull.

Newton's third law begins to answer the question: What gives rise to a force in the first place?Forces are present or arise any time two objects interact with each other. As an example, consider the

situation where a person is pushing a box with one hand. There is an interaction between the hand and

the box because these two objects are in contactwith each other. As a result of this interaction, the

hand exerts a force on the box and the box exerts a force on the hand. These contact forces, which are

normal (perpendicular) to the contact surface of each object, are the result of a complex physical

mechanismbetween the electrons at or near the surface of both objects. This physical mechanism is

what we call thecontact interactionbetween these two objects.
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Check Your Understanding

1. Consider the interaction depicted below between foot A, ball B, and foot C. The three objectsinteract simultaneously (at the same time). Identify the two pairs of action-reaction forces. Use

the notation "foot A", "foot C", and "ball B" in your statements.

2. Identify at least six pairs of action-reaction force pairs in the following diagram.

ANSWERS:

1)

The first pair of action-reaction force pairs is: foot A pushes ball B to the right; and ball B pushes

foot A to the left. The second pair of action-reaction force pairs is: foot C pushes ball B to the

left; and ball B pushes foot C to the right.

2)

The elephant's feet push backward on the ground; the ground pushes forward on its feet. The

right end of the right rope pulls leftward on the elephant's body; its body pulls rightward on the

right end of the right rope. The left end of the right rope pulls rightward on the man; the man

pulls leftward on the left end of the right rope. The right end of the left rope pulls leftward on

the man; the man pulls rightward on the right end of the left rope. The tractor pulls leftward on

the right end of the left rope; the left end of the left rope pulls rightward on the tractor. etc.,

etc.

newton 3rd law of motion

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