Why does an object remains in its state of constant motion if there are no forces acting on that object?

My understanding is that all the energy of the motion will be kept inside and a change in the speed needs a change in the amount of the energy stored by that object. But how is that energy stored in a form of speed? Does anything changes inside the object's structure if it is moving?


2 Answers 2


the main reason is that the space is uniform, and that there is nor absolute reference point in the universe.

basically, what appears to be moving at constant speed to you, will be moving at a different constant speed or even not moving at all to another observer who is moving at constant speed in reference to you. since your point of view is not any better than his, then who said that the object is moving at all? it might as well be staying in the same place.

  • $\begingroup$ I agree with you on this, but my question is more about the object itself. How does it stores the motion energy? I mean, we could be travelling at the speed of light right now (or very close to it) relative to something and not even knowing it. Is it really no reference to changes of the object structure? $\endgroup$
    – user43647
    Apr 1, 2014 at 14:14
  • 2
    $\begingroup$ @user43647, motion is not a property of an object, motion is a relationship between two objects, i.e., two objects can be in motion relative to each other but neither can be said to be absolutely moving. $\endgroup$ Apr 1, 2014 at 14:26
  • $\begingroup$ like @AlfredCentauri pointed out: how do you know that what you think is moving is actually moving? if i'm moving at the speed with the object, i'll claim that it's not moving at all. who's right? $\endgroup$ Apr 1, 2014 at 14:31
  • $\begingroup$ @AlfredCentauri this is probably overcomplicating things for the level of this question, but I'd argue that massless particles are absolutely moving, since every observer measures the same velocity. $\endgroup$
    – Kyle Oman
    Apr 1, 2014 at 19:05
  • $\begingroup$ @Kyle, an invariant speed is not the same as absolute motion. The existence of absolute motion implies the existence of an absolute rest frame of reference to which absolutely moving objects are moving with respect to. However, the existence of an invariant speed does not imply the existence of an absolute rest frame of reference but, rather, the lack of a reference frame for massless objects. $\endgroup$ Apr 1, 2014 at 19:44

What is force? It is the transfer of momentum with respect to time. Momentum is the product of mass of a body, and it relative velocity with respect to something. If we imagine, for once, our world made up of tiny uniform particles, then we can take the mass of one particle as one unit mass. This simplifies the situation, as the momentum, 'mass x velocity' becomes, velocity x 1 = velocity. So, when two such particles collide, they exert force on each other. Or, literally, the faster particle loses a bit of its velocity and the slower particle gains the same with respect to time.

You are asking- " why does an object remains in a state of constant motion if no force acts on it".

This question is same as asking " Why does a unit mass particle has a constant velocity if it is not gaining some velocity."

For some reason, relative motion of a particle in space is a conserved constant. A particle does not gains velocity until some other particle loses the same amount of it. This is the same as the fact that a shopkeeper gains money if some customer loses the same amount of money in his shop.

(Yes, of course money gets lost sometime, or damaged, with a net lose to humanity, but in the universe at least, it is not easy to lose your momentum with nobody gaining the same amount, instantly!)


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