Newton's third law

If every action has an equal and opposite reaction why does a ball fly off after applying a force to it with your foot? And what will happen to the foot, does it move backwards?

Reply 1

Mayhem™

Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

The external force in this instance, is the foot.

Spoiler

Reply 2

Jpw1097

Original post by Jennifer50

If every action has an equal and opposite reaction why does a ball fly off after applying a force to it with your foot? And what will happen to the foot, does it move backwards?

Suppose there are two bodies A and B. Newton's Third Law states that if body A exerts a force on body B, then body B exerts a force that is equal in magnitude but opposite in direction on A.

When your foot makes contact with a ball when you kick it you exert a force on the ball. However, the ball will also exert an equal and opposite force on your foot. This wouldn't necessarily mean that your foot would move back as your foot already has velocity and so the force applied on your foot will cause your foot to decelerate.

Consider walking. When you walk you exert a force on the Earth while the Earth exerts a force that is equal and opposite on you - causing you to move.

Reply 3

Jpw1097

Original post by Mayhem™

Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

The external force in this instance, is the foot.

Spoiler

The question is about Newton's third law.

Reply 4

Jpw1097

Original post by Jennifer50

If every action has an equal and opposite reaction why does a ball fly off after applying a force to it with your foot? And what will happen to the foot, does it move backwards?

You could also approach this scenario using conservation of momentum. Since the ball has a much smaller mass, it will have a greater velocity; likewise since the ball has a much larger mass, in order for momentum to be conserved it must have a smaller velocity.

Reply 5

Jennifer50

So say if you were pushing a light and a heavy table across the ground of the same surface. Why is it easier to move the lighter one than the heavier one?