# Homework Help: Question about drag force

1. Jul 5, 2010

### sona1177

What is the drag force on a 1.6 m wide, 1.4 high car traveling at

A) 10 m/s B) 30 m/s

I'm getting 68 N for part a and 615 N for part B using equation D=.25pAv^2

P= density of air (1.22 kg/m^3) and A is cross sectional area of obect as it faces the wind). *I plugged in 1.22 for p, 2.24 for A, and 30 for v but this is wrong according to the book. What am I doing wrong?

And I don't think I need it to solve this problem but *v terminal = sq rt (4mg/pA).

2. Jul 5, 2010

### PhanthomJay

I don't know how you can solve this problem without knowing the drag coefficient (C_d)for the car. The formula you note is for certain shapes with C_d =0.5; a car could have a greater or lesser C_d depending on its design. (Note: The terminal velocity equation you have noted is for a falling object with a C_d =0.5; it doesn't apply to objects traveling on a level surface.)

3. Jul 5, 2010

### sona1177

my book says to consider Cd as .5 so ive already included it in the equation. The formula is actually .5CdpAv^2 but my books says Cd is .5 so the equation becomes D=.25pAv^2 so is my answer right? thanks

4. Jul 5, 2010

### PhanthomJay

Looks OK to me. Maybe it's a significant figure thing.

5. Jul 5, 2010

### sona1177

Thank you kindly for all your help!

6. Feb 6, 2011

### rebprice

I'm stuck on the same problem about a car that is 1.6 meter wide and 1.4 meters tall and I have to find the drag force for it at 10 m/s and 30 m/s. But I got the wrong answer when I plugged it into the equation D= .25(rho)(Area)(Velocity^2). I found out that the velocity of Air is 1.29 kg/m^3 and plugged that in for rho but still no luck. Help!!!

7. Feb 7, 2011

### PhanthomJay

Assuming quadratic drag and that Cd/2 = 0.25,, the equation is correct, where rho is the mass density of air. I don't know what the book assumed or gave for the drag coefficient.....a mack truck would have a much greater drag cofficient than one of those modern 'bird shaped' vehicles.