# Is it possible to tell whether the space ship is moving or not?

Consider a space ship which is not under any force. Being inside the space ship, I will make a robotic fly from the platform to crawl inside using my remote. For simplicity assume that space ship will be drifting at uniform speed only under forward direction with respect to me, or else it will be under rest. The following experiment is assumed to be done in vaccum.

From the top, the first image is of space ship. Assume that I am at point A and the robotic fly to be beside me as shown.

In the second image, I will make the insect just to lift up and I will stop its engine, as the robot is not under any force it will not fall down, it will stay where it was. As the fly will continue to be under same motion as the space ship, I will not be able to tell whether my ship is moving or not.

In the third image and fourth image, I will apply force to the robotic fly to move little back and I will stop its engine.

• (Third image) If the ship was under forward motion, the fly, because of the applied force backward i.e against its acquired motion from the spaceship, the forward motion of fly will not be synchronzed with the forward motion of the ship, so it will crash the wall with in an interval of time. This ensures that space ship was under uniform velocity.

• (Fourth image) If the ship was at absolute rest, the fly, even though there was an force applied in the backward direction, the fly will not crash with the backside of the space ship. This ensures that space ship was at absolute rest.

But, according to Feynman:

The principle of relativity was first stated by newton, in one of his corollaries to the laws of motion: "the motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line." This means, for example, that if a space ship is drifting along at a uniform speed, all experiments performed in the space ship and all the phenomena in the space ship will appear the same as if the ship were not moving, provided, of course, that one does not look outside...........in a moving space ship the electrical and optical phenomena should be different from those in a stationary ship.

Hoping that my above experiment is neither optical nor electrical, my experiment is in contradiction with the Feyman's explanation. From my experiment, I am able to say whether the space ship is moving or not. How is it possible?

• The same question was discussed in Physics forum. See:physicsforums.com/showthread.php?t=756089. I have generalized the situation here. Jun 2, 2014 at 7:32
• "If the ship was at absolute rest, the fly, even though there was an force applied in the backward direction, the fly will not crash with the backside of the space ship" - can you explain how you came to that conclusion? Jun 2, 2014 at 7:35
• Because the ship was at absolute rest before, the fly will continue to be at rest, even if you make the fly move backward and stop the engine, the fly will remain synchronized with the ship, so it will not crash. Jun 2, 2014 at 7:40
• If the ship is at rest then the fly is also at rest. Then you apply a force to it. What happens to an object at rest if you apply a force? Jun 2, 2014 at 7:45
• Please notice that I am using robotic fly. I can apply the force by starting the engine and stop it by switching off the engine. Now, I hope its clear. Jun 2, 2014 at 8:04

You seem to be totally skimming by conservation of momentum.

Let's look at your situation initially, if you look from there frame it does not matter whether they are at rest or moving with constant velocity.

Let's say the mass of small robot(blue circle) is $m$ and mass of big spaceship(large rectangle) is $M$ ($m \lt M$)

If you just see in there frame, then to move the robot forward you have to push the spaceship back otherwise momentum will not be conserved and it needs to be conserved since there is no external force.

Here we get $$mv_1 = Mv_2$$ $$v_2 = \frac{m}{M}v_1$$

If you see velocity of robot with respect to ship we get $$v_{rel} = v_1-v_2$$ $$v_{rel}=v1-\frac{m}{M}v_1$$

A positive velocity implies velocity towards the front wall which implies that the robot will crash in the front wall

Now if you change the velocity of robot by any means, still the momentum has to be conserved, so again we get

$$mv_3 = -Mv_4$$ $$v_3 = -\frac{M}{m}v_4$$

This time relative velocity will be

$$v_{rel} = v_3+v_4$$ $$v_{rel} = -v_4(\frac{M-m}{m})$$

A negative velocity implies it will collide in the back wall!

No matter how you achieve these velocities or any other, due to conservation of momentum it would not matter whether the "spaceship" was moving or not! And since it does not matter whether the spaceship is moving or not you cannot even tell whether it's moving or not.

If you make the fly move then that requires a force. This force will not be alone, it will have it's pair that would act on the ship. Since ship is massive it would gain lesser velocity than the fly and the fly would crash.

Now if you want to stop this moving fly, then again you need a force which would be equal to the previously applied force just in the opposite direction. This force two will have its pair which would act on the ship. This pair of forces will bring both fly and ship to halt and now fly will not collide with any wall.

• Thank you for the answer. +1 for your effort. I will reply soon. Jun 2, 2014 at 12:37
• If you make the fly move then that requires a force. This force will not be alone, it will have it's pair that would act on the ship. Minor point: if you use a rocket and the ship is open, then no force will act on the ship. Jun 2, 2014 at 21:34
• @Jinawee : That's absolutely true but I have absolutely no idea of a single space craft which people will build without one wall (open). The question as it stands atleast seems to be that the spaceship is closed unless it has no back! I did not find the need to unnecessarily complicate and extend my answer. Jun 3, 2014 at 6:02

If the ship is at rest and you apply a force to the fly, it will crash against the wall.

If you make the fly stop, you have to apply a force in the opposite direction. In the moving frame, this would mean the have the same speed again.

• "If the ship is at rest and you apply a force to the fly, it will crash against the wall." Why? Jun 2, 2014 at 9:03
• @Godparticle If you start the engine, it starts to move. If you stop the engine, it continues to move until it crashes. Law of inertia. Jun 2, 2014 at 9:07
• Switching off the engine might be confusing, Assume that I have applied brake or force to bring it to halt. Jun 2, 2014 at 9:14
• @Godparticle If it halts, then in the moving frame it won't crash. As I said in my answer. Jun 2, 2014 at 9:20
• I am stuck with my own reasons, I am thinking about some obstacles. I will reply if possible. But, +1 for your effort. Thank you. Jun 2, 2014 at 9:53

Godparticle,

It's difficult to refer to your drawings because they are blurred and pale.

However, I think that your confusion might come from the fact that you do not specify (and seem not to take into account) whether there are other forces (than its own engine controlled by you) acting on the fly, like the ones present on earth. So is there any resistance to the motion instilled by air or gravitation?

If there is no other resisting force, and the fly is at rest relative to the ship then it is moving relative to you already, before you even touch your remote. If you make it move up it will still move relative to you because it was given this velocity by the ship before and there is no force to slow it down. If you then make it move forward, than it will have double motion (relative to you) - the one created by the ship (inside which it is located), and the one you gave it to through your remote. Then it will move both relative to you and relative to the ship. In this case it will crash. But if you stop the engine, and there are no resisting forces then it will keep moving relative to the ship (on Earth it would slow down and stop because of resisting forces). If you want to make it be at rest relative to the ship, you need to slow it down to ships velocity (relative to you) but not to zero relative to you.

This is all I can tell based on your description.