# Free fall and gravity

1. Sep 12, 2011

### rajeshmarndi

does a person feel weightlessness or any differences when he make a free fall under gravity of different values say 10m/s^2 or 25 m/s^2.

Also it is said a person float in empty space is similar to a free fall. Free fall means it speed accelerates, then it would reach an infinite speed if it remain in free fall.

2. Sep 12, 2011

### rajeshmarndi

also can a person tell the differences if he is dropped in a free fall of 10 m/s^2 and 25 m/s^2 with his eye blind folded and can he tell that he is falling. I understand its the air resistance that gives him the drag, so assuming no air, only gravity.

3. Sep 12, 2011

### A.T.

No. Free fall means zero proper acceleration (you feel weightless). This is a frame independent (absolute) measure and has nothing to do with speed or coordinate acceleration which depend on the reference frame.

4. Sep 12, 2011

### Naty1

person feels weightlessness...that is what FREEFALL is...zero acceleration locally as AT posted....but to a distant observer it would appear the individual IS accelerating....which would be a coordinate acceleration.

5. Sep 12, 2011

### ghwellsjr

Floating in space is identical to free fall, no matter what gravitational body is nearby that is giving you a coordinate acceleration, but it cannot increase your coordinate speed to infinity because you will either impact that gravitational body, or go into an elliptical orbit around it where your coordinate speed, acceleration, and direction or constantly changing, but if you are blind folded, you will feel like you are just floating stationary in space.

6. Sep 12, 2011

### bahamagreen

I don't think floating in space is identical to free fall.
In floating, none of the constituent parts of the body are feeling a net tug in any direction (except self gravitation).
In free fall the constituent parts are feeling different amounts of tug because the external gravitational field has a gradient.

Floating in space, a non-spinning sphere of point masses would decrease radius while maintaining a spherical shape.

In free fall the same sphere would take on an egg shaped surface as it shrunk (with the pointy end of the egg pointing toward the gravitational source).

7. Sep 12, 2011

### ghwellsjr

Do you think this is something that a person would feel?

8. Sep 12, 2011

### bahamagreen

Not unless the gradient was quite strong. Neutron star, black hole... I've seen the word "spaghettification" used to describe the extreme effect, which a person would feel before being pulled apart...

9. Sep 12, 2011

### Staff: Mentor

This is correct, what you describe is called tidal forces. The equivalence principle equates freefall to a uniform gravitational field. Real gravitational fields are non-uniform, so there are tidal forces.

In GR, tidal effects are caused by the curvature of spacetime. For any curved manifold, if you take a small enough piece of the manifold it is approximately flat (to first order). Equivalently, for any real gravitational field if you take a small enough region of spacetime there is no difference between free fall and floating (to first order).

10. Sep 13, 2011

### A.T.

Unless you build a uniform density sphere with a non-concentric spherical cavity.

11. Sep 13, 2011

### Staff: Mentor

OK, I should have said real gravitational fields are typically non-uniform.