# Homework Help: Air Resistance for A Body

1. Nov 5, 2007

### Br0kenS0cial

I can't seem to come up with a decent formula/accepted value for air resistance.

I realize there are things that need to be taken into account, but I am already delving into things way beyond what I've done per the course work, and I don't want to bite off more than I can chew. I understand there is no exact formula for air resistance, but I need a number to find a terminal velocity.

Can anybody give me a ball park figure for an averaged sized person? (m=70kg, A=1.60m)

2. Nov 5, 2007

### Astronuc

Staff Emeritus
Last edited by a moderator: May 3, 2017
3. Nov 5, 2007

### Br0kenS0cial

Oh, right. I forgot to include that I was using a value of 0.7 for my drag coefficient. Sorry.

4. Nov 5, 2007

5. Nov 5, 2007

### Br0kenS0cial

Er. Thanks for the help, you two, but I really haven't got an answer for this.

Is there one?

6. Nov 5, 2007

### Astronuc

Staff Emeritus
Well what happens at terminal velocity? It means that the velocity doesn't increase, othewise it wouldn't be terminal velocity. If velocity doesn't increase, there is no acceleration, and that means that the net force of the falling object is zero, which means the Drag Force, D, must equal the weight of falling object.

So D = mg, and looking at the NASA site $$D\,=\,C_d\frac{\rho{V^2}A}{2}$$,

where $\rho$ is the density of air, V is the velocity of the object in the air, A is the cross-sectional area normal to the flow (or trajectory), and Cd is the drag coefficient.

So combining the two equations

$$mg\,=\,C_d\frac{\rho{V^2}A}{2}$$.

The rearrange terms and one gets V2 = . . . . . , and V = terminal velocity.

7. Nov 5, 2007

### Br0kenS0cial

hahah that's just my problem, I'm looking for p.

8. Nov 5, 2007

### hotcommodity

Think of what happens to the equation in my link when t goes to infinity. And that equation will give you everything you need to find the terminal velocity.

9. Nov 5, 2007

### Astronuc

Staff Emeritus
Last edited by a moderator: May 3, 2017