Gravity on the Moon vs. on the spinning Earth

[jon2]

TL;DR

moon vs earth gravity

"If the Earth were not spinning, you would be heavier as you would feel the full force of gravity."so how does this apply to the moon? the moon does not rotate and therefore you would experience the full force of gravity as stated above,yes,,yet the appolo missions film footage showed weightlessness ,the moon does not rotate so there should be full gravity experience..yes?no floating as the astronauts appear to be experiencing,,am i missing something here? thanks..j

 

[FactChecker]

The Earth has 81 times more mass than the Moon, so what you see in Apollo is not "weightlessness", [EDIT] it is just one-eightieth of the weight on Earth. That looks like weightlessness but it is not.

[EDIT] @Ibix points out below that the smaller diameter of the Moon makes the person's weight on the surface 1/6 of the weight on Earth, not 1/81.

 

[Dale]

In addition to what @FactChecker said, the moon does rotate. It rotates about once per month.

 

[Ibix]

the moon does not rotate

Yes it does - how else would it keep one face always towards the Earth?

yet the appolo missions film footage showed weightlessness

Not when they're landed on the moon. They have reduced weight but not zero weight.

it is just one-eightieth of the weight on Earth.

Remember that the moon is a lot smaller than Earth too. Surface gravity is about a sixth of a g, not an eightieth

 

[FactChecker]

Remember that the moon is a lot smaller than Earth too. Surface gravity is about a sixth of a g, not an eightieth

Good point. I'm surprised at the huge difference it makes, but I was thinking that 1/81 seemed way too small. 1/6 is a lot more believable.

 

[Ibix]

Good point. I'm surprised at the huge difference it makes, but I was thinking that 1/81 seemed way too small. 1/6 is a lot more believable.

$1/r^2$, remember, and the moon is about a quarter the radius of the Earth.

 

[Ibix]

"If the Earth were not spinning, you would be heavier as you would feel the full force of gravity."

...and finally, remember that the rotation of the Earth makes only a tiny difference to your weight - you'd weigh about 0.3% more at the poles compared to the equator, if my back of the envelope calculation is correct. That's less than the weight difference from eating lunch. If the Earth didn't spin you'd never notice the difference in your weight compared to reality.

Hal Clements' "Mission of Gravity" is an excellent hard scifi exploration of a world with a serious rotation rate, if you ever come across it (the slide rules date it a bit...).

 

[jon2]

In addition to what @FactChecker said, the moon does rotate. It rotates about once per month.

So in turn the moon has gravity because of this slight spin?

 

[Vanadium 50]

No. The moon would have gravity even if it didn't spin.

 

[Dale]

So in turn the moon has gravity because of this slight spin?

No, the spin of the moon is so slight that it is essentially irrelevant

 

[Ibix]

So in turn the moon has gravity because of this slight spin?

No. It has gravity because it has mass - anything with mass has gravity, although it's tiny for anything smaller than an asteroid. The spin just gives you a centrifugal force (same thing that presses you against the car door when you go round a corner) that modifies your weight slightly at the equator compared to the poles.