# Perpetual motion and Newton's first law

This might be a very stupid question. But let me still ask. :)

Perpetual motion is not possible. But by Newton's first law, a body remains in its state of motion unless an external force acts on it.

Now, I wonder if the statement "perpetual motion is not possible" is true only in a practical context, or even theoretically? I remember reading one of the initial chapters on Feynman lectures, that to me, sounded like perpetual motion is not possible even in theory.

So assuming an ideal world of no friction, I roll a ball with some force. And then stop the force. It should keep moving at the same velocity, right? Isn't that perpetual motion?

I know I have some notion that is completely wrong. But can't figure out what. I appreciate any help in clarifying the same.

Thanks!

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## Answers and Replies

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Ryan_m_b
Staff Emeritus
To clarify moving forever and perpetual motion machines (AKA free energy) are two different things. If you had a hypothetically infinite vaccum tunnel and pushed a ball down it it would travel indefinately. The moment you try to get any energy out of it though you will slow it down and eventually stop it.

HallsofIvy