Science Questions

Every action has an equal and opposite reaction, so how much force is produced by urination? Could it knock someone over? Will in Missouri

We've had a few answers sent in for this question. Thanks to everyone for their input!

Brian in the Costa Blanca - Yes you're pee could knock you over. I turned round once and hit someone with my stream and the guy fell over!

Andrew in Brighton - If you're on a travelator that started with a bang or stopped with a bang, you'd fall over if you're going too fast. I looked on the web and the Department of Transport say that half a metre a second is a safe speed, so with regards to the question, if suddenly somebody started up peeing or stops, you'd probably fall over. What you're after is moving someone with a jolt of half a metre a second. As Will said, it's a matter of action and reaction. I'm 85 kilograms. Now if I was going to move backwards at half a metre a second, I would have to project one kilogram of water at 42.5 metres per second. That is a 150 kilometres per hour. If you could do that then you could pee 90 metres in the air, which I would be hard pushed to do. You're talking about a fireman's hose with something like 2kW of power put into it to do anything like that height. If you run upstairs briefly if you're really fit and expend about 1 kW for a short time. That's using the large muscles of your legs, but the abdominal muscles are not designed to do that kind of work at all, so the answer is definitely not. You could not expel urine that fast.

February 2007