2
$\begingroup$

All kinds of movement occur when a thing throws something out or pushes something back and then the thing moves.

Like the car pushes the road back, the rockets throw gases at high speed to move. Everything works on conservation of linear momentum.

Is there any way that a closed system can move without throwing or pushing anything outside of it? Inside of it anything can happen, like wobbling, or internal zig zag movement which can move it.

Note: Throwing or shining light doesn't count.

Is such a system possible, or is it known to modern science?


Edit: I'm clarifying the question to avoid a trivial solution of uniform motion. Which this answer addresses.

Of course, an unaccelerated object moving at a constant velocity will move from point A to B. But that is not what I'm trying to ask.

Let's say an object is at point A, at rest. The goal is to move it to point B, which is at a non zero distance 'd' from point A. So the object must move from point A to point B and then rest in point B. Can this be done by the object itself without it throwing or pushing something outside the object? Is such a thing possible?

The momentum is conserved totally, as the object is at rest in B, as it was in A. But it's initial position is different from it's final position. So this should be theoretically possible as the total momentum is conserved. But how do you get it from point A to B? A small jerk like motion is possible but that length of travel is restricted by the length of the object itself. So please consider that if the width and length of the object is x, let's say the distance that needs to be travelled 'd' should be around 100x. So the initial gain made by some sort a jerky motion should be repeatable to reach 100x distance.

Also please note don't go into unproven conjectures of special relativity or quantum mechanics. Please restrict the answer to classical physics. I'm trying to imagine a macro world classical physics example, like a mechanism of wobbling or a combination of levers pushing and pulling which can kinds of move by a system by itself.

$\endgroup$
1
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Chris
    May 8 at 18:49

5 Answers 5

5
$\begingroup$

Is there any way that a closed system can move without throwing or pushing anything outside of it?

Yes, it can move at a constant velocity without throwing or pushing anything outside of it.

Edit: in response to your edits it is possible to “swim” through spacetime. See https://www.science.org/doi/10.1126/science.1081406 This does not change your momentum, just your position

In the future please do not edit questions to invalidate received answers.

$\endgroup$
0
4
$\begingroup$

Please restrict the answer to classical physics.

The center of mass of your object cannot change its momentum without transferring momentum to or from some other object.

You could make the object wobble or twist if it, for example, contains an internal flywheel, and you turn the outer shell of the object one way while the flywheel inside turns the other way. But the center of mass won't change momentum without some other object to exchange momentum with.

The momentum is conserved totally, as the object is at rest in B as it was in A. ... So this should be theoretically possible as the total momentum is conserved.

Momentum isn't just conserved between the beginning and end of a process, it's conserved at every moment in between as well.

$\endgroup$
1
  • $\begingroup$ Thank you, for a clear answer $\endgroup$ May 8 at 16:13
3
$\begingroup$

It sounds like what you describe is a reactionless drive. That is a hypothetical method of propulsion which does not rely on conservation of momentum. Such a thing is not believed to be possible.

$\endgroup$
2
$\begingroup$

Classically, the short answer is no as you've expressly asked to ignore the most obvious solution, an object moving at uniform velocity.

This means the object must at some point accelerate (otherwise it will be moving at uniform velocity). And this in turn means by Newtons second law a net force must be exerted.

$\endgroup$
2
  • $\begingroup$ Yes but is there a way somehow as to contain the reaction withinthe system and let only action to express to make such a motion possible. $\endgroup$ May 8 at 16:10
  • 1
    $\begingroup$ @HerbertWhitmore: Not as far as classical mechanics is concerned. $\endgroup$ May 8 at 16:12
2
$\begingroup$

Let's say you are in space, in your space gym, which is simply a big lightweight inflatable ball. Inside the ball is you, some nice air, and your gym equipment.

You can move the space gym by grabbing all your weights and kicking the wall, then grabbing the opposite wall when you hit it. The gym moves a lot one way, and you and the weights move a little bit the other way. Somebody outside the gym would just see the gym suddenly move then stop.

You cannot use this method to move the gym by further than its own width. You can't keep it going in the same direction continuously, because it's impossible to get the weights back to their original position without also moving the gym back to its original position.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.