Barry Rawlins asked:
My son Joshua and I were wondering if Usain Bolt was to go sprinting on the moon, he’s the guy who’s this amazing athlete from the Caribbean who seems to be able to run faster than anyone else ever thought was possible. If he was to go sprinting on the moon but without being hindered by extra weight like space suits and equipment and so on, would he run faster or slower than on earth? I believe gravity is a bit weaker, but I’m not sure if that would be a benefit for locomotion or not?
Dave - On the moon, the gravity is about a sixth of the earth so you can jump much, much higher. Whether this helps you with running, it depends on what kind of running you’re doing, I think. If you’re trying to sprint, if the sixth amount of gravity then you’re going to have a sixth of the amount of friction between your feet and the floor because friction basically goes at how hard you’re pushing against the floor, and this means – but your mass is still the same, so you still need the same force to accelerate. So, he’ll be able to accelerate about a sixth of the rate as he could do normally. So, in a 100-meter sprint, he’s almost certainly going to be a lot slower. But if you’re running a very long way, you could probably get an advantage because you can take huge strides. So, you can sort of – you could take a huge stride and then not do anything for a three or four seconds while you fly through the air and then you can land and do a little bit of exercise and fly through the air for a bit. So, you get some time to recover in between so I think you could probably run long distances faster, but short distance is not maybe as quickly.
Diana - So, it’d be like the laziest race ever then, wouldn’t it basically?
Dave - Oh, it depends how fast you’re going but, yeah.
Rawlins, Barry G asked the Naked Scientists: Dear Chris and team, My son and I were wondering: if Usain Bolt could sprint on the moon, without being hindered by extra weight - space suit or breathing equipment - would he run faster or slower than on earth? I believe gravity is weaker, but we're not sure if this is a benefit for locomotion? Many thanks, Barry and Joshua Nottingham, UK What do you think? Rawlins, Barry G , Sun, 6th Sep 2009
Our limbs ( or those of champion sprinters) are fairly optimal for Earth conditions. I think the problems of matching our limbs to the Moon gravity could limit top speed. Both acceleration (traction) and top speed would be affected. You would need to lean much further forward to keep from rotating backwards (something we do automatically down here) if you want good acceleration. This would be difficult or even impossible. As for speed; you would be effectively in a lower 'gear' than optimal.
In theory, if the legs have to support less weight then they should be able to put more effort into moving. In practice though, traction might be a problem. I also recently heard someone talking on the radio about whether it is possible to bring the sprint times down much further because as the speed increases the runner spends less time with their feet in contact with the ground: to go faster you need to apply more force to the ground but as you go faster you have less time to do so. We also need to remember that it's not just a case of strength either; power-lifters have immensely strong legs but don't make good sprinters because they can't move their legs quickly enough, so a sprinters leg muscles not only need to deliver the necessary force but do so quickly. LeeE, Sun, 6th Sep 2009
I agree with both the above. The astronauts who landed on the moon had a different way of moving which was a kind of skip. They also had a huge amount of mass (at least it would have weighed a lot on earth) on their backs and were in cumbersome space suits. They found this to be a better way of getting about. It was also important that they didn't lose their balance because getting up with the suits they had on would have been difficult.
There is a limiting speed corresponding to how fast you can move your leg backwards. I don't think it is all that fast - as people trying to jump/run off an old Routemaster Bus in motion or trying to run downhill will find. lyner, Tue, 8th Sep 2009