# Accelerating Gravity

1. Oct 23, 2005

### Caesar_Rahil

Is acceleration due to Gravity true?
I know it is a simple question, but I have other questions based on this.

2. Oct 23, 2005

### masudr

No.

(I have to make my message longer than 10 characters).

3. Oct 23, 2005

### HallsofIvy

I would suggest to both of you that you climb onto the roof of your house (if you live in a high-rise apartment building so much the better!), step off the roof and see whether or not you are accelerated!

4. Oct 23, 2005

### moose

There is a reason why we say the acceleration due to gravity on earth is 9.8m/s^2

Unless I am completely missing the point.....

5. Oct 23, 2005

### Aether

Would this be a test of Newton's theory, Darwin's, or both?

6. Oct 23, 2005

### pervect

Staff Emeritus
Is philosphy real? :-)

7. Oct 23, 2005

### robphy

This is not a well-posed question.
In the spirit of this SR/GR forum... According to GR...
• after you step off your roof, you are travelling inertially... that is, you are in freefall, traveling along a spacetime geodesic... your 4-acceleration is zero.
• when you stand on your roof [or after you hit the ground], you are not travelling inertially, not traveling along a spacetime geodesic, and have a non-zero 4-acceleration.

8. Oct 23, 2005

### pmb_phy

O-Man! Come on! I can't answer that! I'm still trying to figure out if I'm real. :rofl:

Pete

9. Oct 23, 2005

### masudr

I would suggest that you do that too, but carry drop several items alongside you. Notice that you and all the objects move alongside each other, regardless of rest mass and see that there is no acceleration, as robphy has pointed out.

10. Oct 23, 2005

### z-component

Now that we have determined that there is acceleration due to gravity, what other questions do you have that you say are based on this fact?

11. Oct 23, 2005

### pmb_phy

It's far too difficult to respond to this question as stated since its unclear as to what is being asked.

I do, however, recommend that you avoid doing HallsofIvy's experiment. :rofl:

I can't say more than robphy has already. He's a sharp man and beat me to what I'd have said.

Pete

12. Oct 24, 2005

### Caesar_Rahil

I never mentioned I have quesions that have to be asked in this forum only.

If a body is travelling with a speed of 99.99% the speed of light(Now don't tell me that's impossible). if it enter a strong gravitation force, say, the sun's wouldn't it be accelerated to a speed faster than light.

13. Oct 24, 2005

### pervect

Staff Emeritus
No, it won't. A simpler problem may illustrate why.

Suppose you have three rockets

A stands still
B moves at .9999c relative to A
---->
C moves at .9999c relative to B
---------->

What is the speed of C relative to A? The answer is given by the relativistic velocity addition formula

(.9999+.9999)/(1+.9999*.9999) = .999999995 times the speed of light.

You can imagine that rocket C is being accelerated either by gravity or a rocket engine (except that it will be hard to find a grav field strong enough to accelerate a rocket to .9999c).

Gravity makes things slightly harder to compute, but the basic principle is the same.