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How fast would I have to run to run over water?

Pippa Lucas, London

To avoid sinking, you need to generate thrust equal to your weight. You generate this by pushing water down and backwards with each step. For a size 9 shoe, you can’t push more than around 3.5 litres of water at a time or you would sink too far and friction with the water would slow you down too much. So that 3.5Kg of water has to be pushed back fast enough to offset your weight.
If you weigh 75kg, my back-of-an-envelope calculation suggests you’ll need to push it back at around 11m/s. Since the water moves back as you push it, you need to go twice as fast as that or you would stand still. So that’s a running speed of almost 80km/hr, which would be quite impossible even in my idealised calculation that ignores things like fluid drag.

Comments: 2

http://sciencefocus.com/qa/how-fast-would-i-have-run-run-over-wa

Tue, 2011-09-27 21:02
MikeG

So a cheetah, which can reach speeds of 120 kmh, should be able to run over water?

yes, but only if the cheetah

Fri, 2011-09-30 09:21
zinkeimer

yes, but only if the cheetah is wearing size 9 shoes.

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