Newton’s Third Law of Motion

PhysicsFall 2012

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 action and reaction forces

Newton’s Third Law: for every action, there is an equal and opposite reaction

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.

Motion of a car on the way to school. As the wheels spin, they grip the road and

push the road backwards. Since forces result from mutual interactions,

the road must also be pushing the wheels forward.

The size of the force on the road equals the size of the force on the wheels (or car)

The direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards).

For every action, there is an equal (in size) and opposite (in direction) reaction.

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?

Trick Question! 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.

Many people are familiar with the fact that a rifle recoils when fired. This recoil is 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.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.

Forces always come in pairs - known as "action-reaction force pairs."

Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statements describing who is pushing on whom and in what direction.

For example, consider the interaction between a baseball bat and a baseball.

The baseball forces the bat to the left; the bat forces the ball to the right.

Together, these two forces exerted upon two different objects form the action-reaction force pair.

Baseball pushes glove leftwards. Other force…. The glove pushes the baseball rightward.

Bowling ball pushes pin leftwards. Other force……. Pin pushes bowling ball rightward.

Enclosed air particles push balloon wall outwards.

Other force….. Balloon wall pushes enclosed air particles

inwards.

Consider the interaction between foot A, ball B, and foot C. The three objects interact 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.

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.

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

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’s Third Law: for every action, there is an equal and opposite reaction