# Need help -- inertia in space

## Main Question or Discussion Point

By Newton's second law of motion - inetia,we know that if we push a body in space it will keep moving forever (if nothing stops it) (am I wrong?) but when I think about conservation of energy law im starting to have problems- lets say that I pushed a body in space,the energy converted into kinetic energy and the body stats moving..i know that nothing stops it but how the energy that I invenst converted to kind of infinity kinetic energy?
hlp?

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Drakkith
Staff Emeritus
That's right. Unless some other force stops it, the object will continue to move forever. The kinetic energy it has is finite though, not infinite, so whether you stop it now or in 10 billion years it will possess the same amount of kinetic energy. There's nothing strange about this, it's simply the fact that you need to apply a force to stop something. If we say that we will never apply a force, then the object will continue on forever. (Though that's completely non-realistic of course, as gravity has an infinite range and will constantly act on it)

anorlunda
Mentor
It sounds like you believe that it requires energy for a body in space to keep moving. That is incorrect. No energy is needed.

On earth, things like cars do need energy to keep moving because of friction, but in an ideal space vacuum, there is no friction.

Newton's first law of motion says, "When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force."

When you pushed it, you gave it a finite amount of kinetic energy. It still has that energy later (assuming nothing else happens to it), and it will have it as long as it keeps moving. This energy lasts as long as the body is moving, but it does not increase. It is the same amount of energy as it was when you had just pushed it. Infinitely lasting is not the same as infinite amount :-)