# Fluid/Air Friction

1. Sep 2, 2006

### Sh00b

I'm trying to figure out the trajectory of a ball but i don't know how to incorporate air friction. I've googled it several times but havn't found anything. Most are just examples but don't describe how to calculate it.

2. Sep 2, 2006

### e(ho0n3

If you have all the forces on the ball, including friction, you can find the net force on it and use that to determine its trajectory. Pretty simple really.

3. Sep 2, 2006

### Sh00b

Well, i got the trajectory of the ball without friction already, but i don't know how to add fluid friction simply because i don't know how to calculate it. Is there some kind of formula for fluid friction?

4. Sep 2, 2006

5. Sep 2, 2006

### Sh00b

So...drag equation is:
D = Cd * r * .5 * V^2 * A
With:
Cd = Drag Coefficient
r = air density
V = velocity
A = reference area

To find the drag coefficient, the only thing it describes "The drag coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and flow conditions on aircraft drag."

So i'm now confused on how to get the drag coefficient.

6. Sep 2, 2006

### Staff: Mentor

Drag coefficients are determined experimentally, and for some common shapes, e.g. a ball or sphere, the drag coefficient may be tabulated and available from a report or textbook.

7. Sep 2, 2006

### desA

Lump all the terms into one 'constant' - you can compute it more accurately later, then you have a relationship in v^2... Work this back into your equation/s of motion & you should be well on your way...

desA

8. Sep 3, 2006

### andrevdh

The direction of the drag opposes the motion of the ball. That is it will be in the opposite direction of the velocity of the ball. Are you trying to solve it numerically or analytically?