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To my understanding, work is done on object A when object B is applying a force on object A, causing object A to displace.

Since energy is the ability to do work, what work does a moving object do, due to its kinetic energy? I mean, if I kick a ball, it has kinetic energy due to its velocity, but what force does it apply? On what object?

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  • $\begingroup$ I think I have a deep misconception on this subject, so I would be grateful to a good explanation, yet not too advanced one (mathematically speaking). $\endgroup$ Commented Apr 26, 2016 at 16:31
  • $\begingroup$ An object in motion will stay in motion unless a force is applied. When the ball hits something, then it will apply a force. Just moving, it doesn't (well, neglecting air resistance). $\endgroup$
    – Jon Custer
    Commented Apr 26, 2016 at 16:34
  • $\begingroup$ It is a good answer, so I think you should write it as one so I could accept it. But a follow-up question: If for example the ball hits a building, then there is no work because there is no displacement. So actually the kinetic energy did not did work. What is the explanation? Thank a lot! $\endgroup$ Commented Apr 26, 2016 at 16:39
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    $\begingroup$ I'm not sure what your question is - you say "energy is the ability to do work" and then you are wondering what work is done by an object with kinetic energy, but having the ability to do something is not the same as doing something. $\endgroup$
    – ACuriousMind
    Commented Apr 26, 2016 at 16:44
  • $\begingroup$ @ACuriousMind - First, I think the title edit is not good because I want to do how the kinetic energy becomes work, end not how kinetic energy works... Secondly - I understand that now after reading Jon Custer's answer. Thanks a lot. :) $\endgroup$ Commented Apr 26, 2016 at 16:48

2 Answers 2

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To my understanding, work is done on object A when object B is applying a force on object A, causing object A to displace.

Work is done whenever a force displaces an object.

Since energy is the ability to do work, what work does a moving object do, due to its kinetic energy?

A moving object might not do any work at all. Imagine an empty universe with just a ball. The ball is moving, meaning it possesses some kinetic energy. If nothing interferes, (and we're on an hypothetical empty universe, so there's nothing to interfere), the ball will just keep moving indeterminately.

I mean, if I kick a ball, it has kinetic energy due to its velocity, but what force does it apply? On what object?

If you kick a ball you're transferring energy to the ball. The ball that was quiet and had zero kinetic energy, now has some kinetic energy, proportional to the square of its velocity.

If you ignore air resistance, it won't be applying force on anything/any object. However, if we consider that the ball is near enough of the Earth, the ball will have a force applied on it: gravity force, in the direction of the center of the Earth. (actually, the ball itself will also be applying a force on Earth, but given the magnitude of the difference between their masses, let's ignore this). And this force from the Earth will be changing the speed of the ball, thus changing its kinetic energy, (transforming it in potential energy in case the ball is going up, and actually providing it with more kinetic energy in case the ball is going down).

If you don't ignore air resistance, the ball will be pushing and displacing the air particles. Meaning it is applying a force and displacing them, thus doing work on them.

So to answer your question, just because an object has kinetic energy, does not mean it is using it. If there is no change of state, there's no "usage".

I sincerely hope my answer helps you understand it better :)

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  • $\begingroup$ It did answer my question. Thanks! But if it hits a wall, the ball will not move, so there is no work done in here... So what is happening here? Thanks again! $\endgroup$ Commented Apr 26, 2016 at 17:03
  • $\begingroup$ I'm glad it helped :) From what I understand of your question, you mean to ask what is the work if when you hit the ball, the ball is against a wall and doesn't move? If that's the case, there's no change of state, there's no work done. And in that case we're talking about Newtown's third law. You might want to do that as a new question ;) $\endgroup$
    – A. Baila
    Commented Apr 26, 2016 at 17:08
  • $\begingroup$ I think I understand the answer from your comment. Tell me if I am right, please. When the ball hits the wall, it cannot do any force that will displace the wall, so instead it will transform to another kind of energy, which could also do that work? I am sorry for my English... :\ $\endgroup$ Commented Apr 26, 2016 at 17:12
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The work the ball that you kick does is the ball going against air-resistance and changing its velocity.

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  • $\begingroup$ At least to me that seems very reasonable $\endgroup$
    – Hadus
    Commented Apr 26, 2016 at 16:37
  • $\begingroup$ But what if there is nor air resistance, like in space? Where can we see the work then? Thanks a lot. $\endgroup$ Commented Apr 26, 2016 at 16:38
  • $\begingroup$ In space the work done is the change in velocity. It takes energy to speed up things for example a rocket even if there is no air-resistance. $\endgroup$
    – Hadus
    Commented Apr 26, 2016 at 16:45
  • $\begingroup$ But work is to displace an object using force, not changing its velocity... $\endgroup$ Commented Apr 26, 2016 at 16:51
  • $\begingroup$ You are displacing the object by making your foot be in the place that the ball was in by doing this you are increasing its velocity. $\endgroup$
    – Hadus
    Commented Apr 26, 2016 at 16:54

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