Why a man on the Moon can jump 21 times higher than on the Earth

[sophiecentaur]

Here a study about vertical jump performance of apes, suggesting that it requires muscle properties significantly different than those of human muscles.

http://rspb.royalsocietypublishing.org/content/273/1598/2177

That's interesting and appears to be a pretty thorough bit of investigation. I am surprised that I couldn't find a comment on the obvious difference between ape and early human lifestyle - humans were runners and not arboreal - probably some of the most effective running hunters ever and seemed to have managed to bring down massive prey by simply exhausting them by running them into the ground. I heard (unspecified radio programme) that it was assumed that both sexes would have needed the same abilities in order for the mums with children could be present at the kill in order to eat the stuff before other predators arrived to steal it.
But I am talking well above my pay grade on this topic, of course. (Standard PF practice, so no apology. )
On the topic of evolution and gravity, whatever the lifestyle of an Earth organism, it will have developed with a fixed value of g and, just as with all other abilities, there is every reason to suspect that some degree of optimisation of all abilities must have taken place. 'Nature' seems to do no more than absolutely necessary, when it comes to abilities. We really don't preform well outside a narrow range of temperatures (without clothes etc.), in non-standard proportions of atmospheric gases or in the presence of unfamiliar microbes. A local g of g/6 is not a trivial difference and I don't see how it can be assumed that we would be well adapted well to it.

 

[Art Vanderlay]

A local g of g/6 is not a trivial difference and I don't see how it can be assumed that we would be well adapted well to it.

Because there is nothing significant about the physics to suggest otherwise. Go and check out a gym and see people doing leg press (any g) or squat (greater g). there are many examples of where we would use our legs to push less than our own weight. A simple experiment on Earth would be to suspend someone from a tether providing a constant upwards force of 5/6g... that's an identical environment to the moon. I'd be surprised if this has not already been done, e.g. in preparing for the moon landings.

Also, I think it is a pretty trivial difference. Do the mechanics of jumping change when you have a heavy bag on you bag? Why should it be so much different in lower g?

 

[sophiecentaur]

Because there is nothing significant about the physics to suggest otherwise.

If you were doing any other experiment and suddenly introduced a factor of 1/6 into the variables then would it be good practice to 'assume' that it would would make no difference? You would just have to consider this as a major factor until you could prove otherwise.
We evolved with our weight and mass being relegated by a constant reducing one but not the other by such a large factor is not trivial until it's proved to be.

a tether providing a constant upwards force of 5/6g.

Why not find out about it and prove me wrong then? Unfortunately, I can't imagine NASA having built a test rig tall enough to cope with high jump records but you never know.
Actually, you wouldn't need NASA facilities. If you could find a high bridge with access top and bottom you could attach a pulley with a 5/6 body weight mass on a rope . .etc.

 

[Art Vanderlay]

If you were doing any other experiment and suddenly introduced a factor of 1/6 into the variables then would it be good practice to 'assume' that it would would make no difference? You would just have to consider this as a major factor until you could prove otherwise.
We evolved with our weight and mass being relegated by a constant reducing one but not the other by such a large factor is not trivial until it's proved to be.

Why not find out about it and prove me wrong then? Unfortunately, I can't imagine NASA having built a test rig tall enough to cope with high jump records but you never know.
Actually, you wouldn't need NASA facilities. If you could find a high bridge with access top and bottom you could attach a pulley with a 5/6 body weight mass on a rope . .etc.

High enough? The amount someone can jump from a standing start is about half a meter. Do you think this is high?

This has become a pointless debate with no new physics. The only unknown is whether the mechanics of the human legs dramatically changes in lower g. Well people have different power to weight ratios here on Earth also - do they jump differently?

Just saying some assumption is not valid with no reasoning is not science. There's no reason to believe it is either more or less efficient to jump on the moon - but it could be either way.

Reply if you wish, but I'm no longer responding on this thread since it appears to be just point-scoring based on blind objections with no physical reasoning.

I've given a reasonable model for the system which I hope others will find valuable.

 

[sophiecentaur]

Just saying some assumption is not valid with no reasoning is not science.

It's the other way round, surely. If you make an assumption then you should validate it. I was introducing a note of caution into making such a simple assumption - after all, there have been more than one approach to the theory, even on this thread.
Of course different people perform differently, under the same conditions. The issue is how differently one particular individual would perform under different conditions. Unless you can justify ignoring some parameter then you should really include it - that's a good scientific practice, isn't it?
I think the scientific reasoning behind my doubt would probably come from reference to the Power / Force graph, which doesn't show a simple relationship. I should have thought that evolution would have optimised the way the legs propel and lift the individual (running and hunting - not in a gym exercise).
It would be an interesting experiment to do before actually going all the way to the Moon.

 

[Art Vanderlay]

It's the other way round, surely. If you make an assumption then you should validate it. I was introducing a note of caution into making such a simple assumption - after all, there have been more than one approach to the theory, even on this thread.
Of course different people perform differently, under the same conditions. The issue is how differently one particular individual would perform under different conditions. Unless you can justify ignoring some parameter then you should really include it - that's a good scientific practice, isn't it?
I think the scientific reasoning behind my doubt would probably come from reference to the Power / Force graph, which doesn't show a simple relationship. I should have thought that evolution would have optimised the way the legs propel and lift the individual (running and hunting - not in a gym exercise).
It would be an interesting experiment to do before actually going all the way to the Moon.

You are just providing reasons why others' analysis is "wrong", rather than offering any yourself. Making assumptions is the way we simplify a problem enough to solve it. You could argue that g isn't constant in either case due to height, you could argue that Newtonian physics is invalid since general relativity more correctly describes reality.

I assume the reason why one might ask this question is to understand how the mechanics of the problem affect the outcome, not to understand if the action of jumping itself is changed. So if you want to continue to speculate about this point, carry on, I'll split the problem into 2:

1/ What height will a mass propelled with the same force over the same distance vertically reach on the moon vs the Earth?
2/ Is the way a person jumps affected on the moon?

Answers:

1/ h'/h = (1 + h/d - r)/(rh/d) where h' is the height on the moon, h is the height on the Earth, d is the distance the force is applied over and r is the ratio of the moon's to Earth's gravity. Anyone looking on this forum for the answer to that question, use the parameters you want for d, h and r and you have h'
2/ Feel free to answer yourself or continue to speculate. It doesn't interest me because it can't be tackled theoretically, it requires empirical evidence. My guess is that the only significant factor is the fact the the contraction speed would need to increase (as I have already analysed). This could in principal change the total force integral. I can't see that the contraction mechanism itself would be affected (you should be able to work out why that is yourself)

Maybe someone else can pick up the debate with you on 2. Good luck.

 

[sophiecentaur]

I assume the reason why one might ask this question is to understand how the mechanics of the problem affect the outcome, not to understand if the action of jumping itself is changed. So if you want to continue to speculate about this point, carry on,

My speculation is totally based on the fact that humans who use machines of any sort tend to use Gears. If you have ever tried to cycle fast in a low gear, you will remember that you are speed limited. The same thing applies to motor cars. Engines and (from the graph in that previous post) muscles have a definite optimum speed for operation. Why should this not apply when trying to jump as high as possible in low g?