Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

We know that a small object moving fast enough can pass by a planet and escape its gravity. Would this be (theoretically) true in reverse? Meaning a planet moving fast enough past a stationary smaller object would not assert its gravitational pull on the small object? Would this mean that gravity is mitigated by velocity regardless of whether the more massive object is moving as opposed to the small object?

share|improve this question
add comment

2 Answers 2

What you're describing is a change of co-ordinate system. For us sitting on Earth it makes sense to choose co-ordinates centred on the Earth and moving with it, because that makes the maths easy. In these co-ordinates we are stationary and the small object is moving. However it makes just as much physical sense to choose a co-ordinate system centred on the object and moving with it. In these co-ordinates the object is stationary and the Earth is moving. The key point to understand is that the physics doesn't care what co-ordinates we use. The interaction between the Earth and the object is the same in both sets of co-ordinates.

Generally in physics it's common to choose whatever set of co-ordinates makes it easier to calculate what happens. For example when the bodies are roughly equal in mass we often choose centre of mass co-ordinates i.e. we measure everything relative to the position of the centre of mass of the system. In general relativity we sometime use different co-ordinates in different parts of our system and patch them together where they join.

So, to answer your question, it makes no difference whether the object is moving and the Earth is stationary or whether the Earth is moving and the object is stationary.

share|improve this answer
John, I find that simply amazing. I'm still trying to wrap my head around relativity. –  Argo Apr 5 '13 at 18:32
Would this make space, that is a location an object occupies, relative? It seems like the what (mass) and the why (physical laws) are fixed, but the where (space...or space time) is not. Is "how much space an object occupies at any given moment" relative to its velocity? I guess what I'm asking is if an object is moving isn't velocity always going to be a factor of "where" it is? –  Argo Apr 5 '13 at 19:48
If I've understood you correctly then yes, spacetime (we can't consider space and time separately) is relative. There is no such thing as an absolute velocity. The velocity depends on the co-ordinate system you use i.e. how you measure spacetime. This seems a complex idea for beginners, but actually it's a very simple and important one. –  John Rennie Apr 6 '13 at 8:10
Would it then be safe to assume that a moving object would...hmm "occupy" less space time relative to a stationary object of equal mass? John, do I have any kind of grasp on this at all? –  Argo Apr 6 '13 at 13:44
I'm not sure what you mean by the question. Are you referring to the length contraction seen in special relaivity? Maybe you should post this as a new question. –  John Rennie Apr 6 '13 at 13:49
show 2 more comments

The smaller object's velocity may be high so that it can escape from falling into the planet. But, it's affected until $\infty$ because the object doesn't (can't) actually neglect the planet's gravity. So, it is deflected from its actual path. Maybe, the deflection is negligible. But, there is always a deflection based on the object's velocity and mass. In this view, the converse is also true.

Recalling SR's postulate that there's no special reference frame (i.e.) The laws of physics are the same for all inertial observers.

So, a stationary object is attracted by the planet. After the planet has passed away, you can notice the motion of the object for sure. Any body under the influence of gravitational field is affected..!

share|improve this answer
Thanks! This actually reminds me of a trite little experiment I did. I took a magnet and zipped it passed a paper clip. The clip did not adhere to the magnet, but it did slide. Just a you explained...it was effected by the magnet, though not directly attracted to it. Again, thanks for taking the time. –  Argo Apr 5 '13 at 17:07
@Zach: Hi Zach. Just minding you that gravity is much much weaker (terribly) than electric & magnetic fields..! ;-) –  Waffle's Crazy Peanut Apr 5 '13 at 17:15
Oh yes. I realize it's not a real great comparison...magnets to gravity. –  Argo Apr 5 '13 at 18:30
add comment

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.