# B Moon gravity and vacuum

1. Aug 2, 2016

### earl tasylor

I read on the net that the moons gravity is one sixth that of earth's so if I could jump 12" off the ground on earth it would be 72" on the moon.

I also see (you tube) that in a vacuum (akin to the moons surface) if you drop a feather and kilo weight simultaneously they reach the ground at the same time.

Does the vacuum have any effect on the first paragrap. If you're jumping through a vacuum could you jump higher as there is less resistance.

2. Aug 2, 2016

### Tazerfish

Air resistance is largely irrelevant in this case.
So no, the vacuum(or the absence of air resistance) has no significant impact on how high you can jump.
When throwing a ball you could expect a decent difference.

3. Aug 2, 2016

### earl tasylor

On that basis you could expect to jump higher, if not 6x then maybe 2-3x for example.

Moving on. The dust from the tyres of the lunar module on Apollo missions seemed to act in earths gravity. If a vacuum isn't countering the moon gravity why does the dust thrown from the tyres act this way.

4. Aug 2, 2016

### A.T.

That is probably an underestimate. The limiting factor would be the velocity-force relationship of the muscles, but there is no obvious reason why the limit should be 6x. See this previous thread:

https://www.physicsforums.com/threa...ump-21-times-higher-than-on-the-earth.774140/

5. Aug 2, 2016

### nasu

What do you mean by "the vacuum countering the moon gravity" ?
The vacuum has no such effect on gravity. You probably misunderstood some basic physics. Maybe you can explain a little more.

6. Aug 2, 2016

### sophiecentaur

This has been discussed at length in several past threads. It's hard to make a good prediction because (at least in my opinion) the speed that your legs can actually push your feet downward is limited by leg length and their actual mass (moment of inertia). I have a feeling that one could do a pretty valid experiment using a bungee cord from a high suspension point which is pulling you upwards enough to remove 5/6 of your weight. So you would be accelerating your full mass against 1/6 of your weight. The lift force from the bungee would not change noticeably if the cord is a few tens of m long (negligible fractional change in length).
I suggest that you could do better if you used 'blades' (as used by amputees ) of an appropriate length and stiffness to match your muscles better to the changed requirements.

7. Aug 2, 2016

### sophiecentaur

You don't mean that, do you? It would go upwards, towards the Earth if it were. The motion of the dust appeared fairly fast but we were visually comparing it with the behaviour of dust through air on Earth. It is easy to make assumptions about what you see under such strange conditions. Everything in those videos of life on the Moon had a slo-mo look about it because of the difficulty in walking normally and it seriously affected our impressions of the dynamics up there. I am more prepared to go with the theory, actually and ignore what I thought I saw.