# In space, an accelerating platform hit my feet from under

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1. Apr 23, 2015

### Ryan Bruch

Suppose I am stationary in space (no velocity). My body is straight. There is a stationary ball right beside me (also no velocity). Then an accelerating platform hit my feet and the ball from under. Now the ball and I are in contact with the platform constantly and accelerate at the same rate as the platform does (is this right?). It would be like I am standing on the platform.

So:

1. If I Jump from the platform, would I land back on the platform?
2. If I pick up the ball on the platform, would I feel that it is heavy?
3. If I pick up the ball from the platform and let go of it, would it land back on the platform?

2. Apr 23, 2015

### Drakkith

Staff Emeritus
1. Yes, you will. Since the platform is accelerating, it will catch back up to you, which is indistinguishable from jumping into the air and falling back to the ground.
2. Yes, because the force exerted by the platform is transferred through your arm to the ball, so you will feel its weight.
3. It would land back on the platform for the same reason that you yourself would land back on the platform after jumping.

3. Apr 24, 2015

### Ryan Bruch

Wait, weren't I be accelerating at the same rate as the platform does after the accelerating platform hits my feet like an elevator?

4. Apr 24, 2015

### Drakkith

Staff Emeritus
You would be accelerating at the same rate as the platform as long as you were standing on it. When you jump and are above it, you are no longer accelerating, so the platform catches up to you.

5. Apr 24, 2015

### Cruz Martinez

You wont keep accelerating once you stop touching the platform due to the law of inertia. That is the same reason you'd feel your own and the ball's weight when standing on the accelerating platform.

6. Apr 24, 2015

### Ryan Bruch

Wouldn't I be remaining in motion per Newton's first law, like how I throw
Wait, I am in space. Isn't my situation the same as this: "If I throw a ball upward while on the roof of a moving train, the ball would land right back in my hand instead of landing farther back"?

7. Apr 24, 2015

### Cruz Martinez

That also happens due to the law of inertia. The law of inertia says that an object on which no forces act keeps a constant velocity along a straight line, in this case the "conserved" velocity is along the direction of motion of the train since no forces act in that direction.

In the case of the accelerating platform, once you jump up and stop touching the platform, your veocity will be constant i.e. you will stop accelerating.

8. Apr 24, 2015

### Ryan Bruch

So if the train in my example is accelerating, the ball would not land right back in my hand? please use "correct" or "incorrect."

9. Apr 24, 2015

### Cruz Martinez

I suppose you mean a moving train accelerating horizontally, in that case yes the ball wouldn't land right back on your hand but on a different spot.

10. Apr 24, 2015

### Ryan Bruch

Okay, I understand now. The condition would be like when I am on Earth, but what about liquid water? Assuming that the platform is covered with a dorm, the temperature is optimal, and the dorm is filled with enough gas so that the pressure is enough for water to become liquid, how would it behave?

11. Apr 24, 2015

### Drakkith

Staff Emeritus
It would behave nearly identically to how it behaves on Earth. I say nearly identically because you don't have tidal forces when accelerating on a platform like you do with gravity. This is the Equivalence Principle as initially stated by Einstein: http://en.wikipedia.org/wiki/Equivalence_principle

So the original equivalence principle, as described by Einstein, concluded that free-fall and inertial motion were physically equivalent. This form of the equivalence principle can be stated as follows. An observer in a windowless room cannot distinguish between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g. This is not strictly true, because massive bodies give rise to tidal effects (caused by variations in the strength and direction of the gravitational field) which are absent from an accelerating spaceship in deep space.

12. Apr 24, 2015

### Ryan Bruch

Would the difference in how it behaves be apparent (can be seen with the naked eyes)?

13. Apr 24, 2015

### Drakkith

Staff Emeritus
There would be no visible difference.