# G-force on ground

1. Aug 1, 2014

### DocZaius

I was looking at this graph and wondering what the value of the graph would be before the engines turn on. Should it be 1 g-force or 0 g-force? (the "force" in g-force being a misnomer as it is a measure of acceleration, not force) Below are arguments for each:

Why it should be 1 g-force: We experience 1 g-force of acceleration on earth's surface. The astronauts would also experience 1 g-force before the engines turn on.

Why it should be 0 g-force: The acceleration of the rocket (in m/s^2) is clearly 0 while the rocket is still on the ground. 0 m/s^2 corresponds to 0 g-force.

Which is the correct interpretation? Or is this just a problem of choosing your zero? In that case what is the convention?

2. Aug 1, 2014

### A.T.

That's "proper acceleration":
http://en.wikipedia.org/wiki/Proper_acceleration

That's "coordiante acceleration" in the rest frame of the surface:
http://en.wikipedia.org/wiki/Acceleration

If it says "G-force" it probably means proper acceleration. Also because it drops to 0 during stage changes. Coordinate acceleration would drop to -1g.

3. Aug 1, 2014

### DocZaius

Ah I should have paid attention to the stage changes! No ambiguity there. Thank you!

4. Aug 1, 2014

### A.T.

And also the figure caption:

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