Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Newton's Third Law states that

Whenever any force is exerted by a body#1 on any other body#2, another force which is equal in magnitude and opposite in direction is exerted on body#1 by body#2.

In theory, suppose a boy is pushing a rock heavier than him on ice. Due to the relative lack of friction, the boy would probably be pushed back more than the rock was pushed forward. How is this possible? How can an inanimate object create a force?

share|cite|improve this question
Why can't an inanimate object create force? When you move your hand across a surface, don't you experience friction? – mikhailcazi Aug 11 '13 at 5:04

How can an inanimate object create a force?

If you were to trip on a banana peel and slam your face into the ground, it would hurt a lot.

Brought to you by the Earth: the inanimate object that just can't let it go.

share|cite|improve this answer

When you walk, you push the ground back with your foot right?

Then how are you moving forward if the force is backwards?

Newton's third law says that If you exert a force on the ground backwards, the ground exerts a force on you forward, of equal magnitude(friction). This force pushes you forward.

There you have another example of an inanimate object exerting a force, without which you could not have existed.

Edit: To summarize all the answers, what we are trying to do is basically convince you that Even inanimate objects can exert a force. The difference between a living object and an inanimate object is that living objects can control how they apply a force to some extent, which is probably the only difference between the two.

share|cite|improve this answer

In a manner of speaking, Newton's declaration is a bit of "stating the obvious".

Let's try to put it in a different context... if you put your palms together and press, each feels exactly the same magnitude of force, just in opposite directions.

The point of his law is to allow you to do analysis of forces in a given system. Consider a building sitting on some patch of ground. The building exerts some force on the ground because of its mass (weight). The ground must resist that force if the building is to continue to stand in place, so the ground exerts an upward force on the building. The forces are exactly equal and opposite. The point is that you can add up the weights of all the parts of the building, together with everything in it, to come up with some total weight. Knowing that, you immediately know how strong the ground must be to hold it up.

The ability to switch perspective from "weight of building" to "force to support building" is what his third law is really all about.

share|cite|improve this answer

How can an inanimate object create a force?

In this particular instance, you don't actually need to think of the situation in terms of reaction forces.

What you have is leg muscles contacting and by means of a ligament around the outside of a joint extending that joint. Your muscle is providing all the force.

That force acts to extend the length of you body. In space, your body would extend in two directions and your centre of mass would remain at the same position in space. On the ground, you have your feet in contact with the ground and your hands in contact with a rock - so the force is exerted equally on your feet and on your hands (you can feel this in both those parts).

What happens next is a question of which object, rock or ice, provides least resistance. If the rock has a large mass it will not move as easily as your feet do on ice.

This is really a horizontal analogue of what happens vertically when you jump. You don't remain still and propel the planet down away from you because it's more massive. It's the work done by your leg muscles that propels you upwards - the planet didn't provide that energy.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.