Asked by: 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 3.5 kg of water has to be pushed back fast enough to offset your weight. If you weigh 75 kg, my back-of-an-envelope calculation suggests you’ll need to push it back at around 11 m/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 80 km/hr, which would be quite impossible even in my idealised calculation that ignores things like fluid drag.