# Why is kinetic friction constant?

1. Oct 5, 2006

### Skhandelwal

I don't get it, first of all, why does air friction quadruples as velocity doubles? Second, if that air friction works in that way, why does kinetic work in a constant manner?

2. Oct 5, 2006

### Andrew Mason

The short answer is that it doesn't, exactly. The formula for kinetic friction is empirically derived and is only approximately true.

Air friction does not always vary as the square of the speed. At slower speeds it is proportional to speed. At higher speeds it is proportional to higher orders of the velocity. Fluid dynamics is very complicated and has to take into consideration many different things that go on when an object moves through a fluid.

AM

3. Oct 6, 2006

### tim_lou

approximately... there is linear drag and quadratic drag. linear drag dominates for small objects (example: a piece of dust), linear drag is caused by aerodynamic flow passing the object...

for big objects, quadratic drag dominates. You can think of it this way, when an big object is moving, it acclerates the air in front of it. Let's say the objects is moving at v and the air is stationary relative to it. for time dt, the object moves a distance of v*dt, so that the number of molecules it hits is around v*dt*(area), the momentum transfered is around m*v so that the the change in momentum per dt ~ m*v*v*dt*(area) ~ v^2. since force is change in momentum per time, the drag goes quadratically...

my explanation is very very VERY rough, do not take it too literally.

Last edited: Oct 6, 2006
4. Oct 7, 2006

### Skhandelwal

all of you are explaning it mathematically, I have problem understanding it conceptually.

5. Oct 21, 2006