Q & A: friction in fluids

Q:
what type of friction is in water? i am aware of water resistance, but i read somewhere on your website that there is more friction when an object moves faster through the water. how is this possible? for example, when a weight is dropped into water, what friction is present then?
- Georgie (age 15)
UK
A:
You're probably already familiar with how friction increases as your speed with respect to a fluid increases. You know that on a windless day you experience a lot more air friction when you're running than when you're walking. This usually holds for friction in any fluid. You can think of it as your momentum leaking away into the fluid. The more momentum you have, the bigger the leak. The physical and mathematical descriptions are a little beyond a short answer, but when you study them you will see that for moderate speeds, they are simple.

The more interesting case is the friction between two sliding surfaces. You probably learned in school that it doesn't depend on how fast the surfaces slide past each other. That's more or less true, but the reasons are complicated, and I can't give them because I don't understand them, not because of space limits.

Mike W.

(published on 05/16/2013)