# Is relative motion different from acceleration?

1. Apr 22, 2013

### VeryConfusedP

I saw someone post a question or a comment about relative motion and acceleration. It got me thinking about the spaceship in deep space with rocket boosters. Well, when that guy or girl hits the booster, he or she accelerates, but the person is not necassarily moving relative to anything.

Is that correct?

I have another question, but I just want to make sure I have this correct.

To clarify: when I say "deep space", I'm talking about a point in space where there is no matter (stars, planets, humans, whatever) that you can see. When you hit that rocket booster, you are accelerating into dark space.

2. Apr 22, 2013

### Mentz114

The rocket will be moving relative to the reaction mass. But proper acceleration is not relative and can be detected in the accelerating rocket ship.

3. Apr 22, 2013

### VeryConfusedP

Mr. Mentz:

When you say "detected", are you implying something about weight? I thought you had to be in a gravitation field to experience weight. Is that spaceship in a gravitational field?

Last edited: Apr 22, 2013
4. Apr 22, 2013

### Mentz114

I didn't use the word 'felt'.

Proper acceleration will cause weight to be detected.

5. Apr 22, 2013

### VeryConfusedP

You are correct. Not sure why I missed that.

6. Apr 22, 2013

### A.T.

coordinate acceleration is relative = accelerating relative to some reference frame
proper acceleration is absolute = accelerating relative to a local inertial frame

When you fire the boosters, your proper acceleration is not zero. You can measure it locally with accelerometer attached to the ship. No external reference is needed to measure it, so we call it "absolute".

Unfortunately we cannot remove the rest of the universe to do an experiment without it. We don't know if proper acceleration was still measurable locally, if the rest of the matter was gone.

7. Apr 22, 2013

### Staff: Mentor

Take a weight and suspend it by six springs from the floor, ceiling, and four walls of a compartment in the spaceship. When the rockets are operating and the ship is accelerating, the springs will stretch in a way that can be measured by an observer inside the ship (and the observer will experience the floor pushing on his feet as well, as there has to be some force accelerating him to keep up with the accelerating ship he's in). This works even if the ship is the only thing in the universe and even if the compartment inside the ship is completely isolated from the rest of the universe - no windows, no way to look out, no way to observe anything outside the ship.

You can't do the same thing about constant motion; there's no device that can detect a constant velocity from inside the closed compartment. If you're going to talk about the velocity of the ship, you need something the ship to compare against.

However... I cannot think of any way of accelerating the ship that won't also create some external object that we can use as a reference for velocity - for example, if you use rockets to accelerate the ship you'll end up with rocket exhaust outside of the ship. So if you're looking for something that's "accelerating but not moving" I don't think you'll find it.