Posted by Heather on .
A projectile of mass 0.800 kg is shot straight up with an initial speed of 28.0 m/s.
(a) How high would it go if there were no air friction?
(b) If the projectile rises to a maximum height of only 26.8 m, determine the magnitude of the average force due to air resistance.
I got the correct answer for part A, it's part B that's giving me trouble. I tried using 1/2 mv2=Fd but it seems that I'm missing something.

Physics 
drwls,
Good work on part (a). I will assume you got 40 meters for the answer
For part (b), there is a potential energy deficit of (40  26.8)*M*g at the top of the trajectory. Set this equal to work done against friction, F*26.8 m, and solve for F.
With your equation, you were assuming that ALL of the initial kinetic energy went into friction work. Only some of it did. 
Physics 
Heather,
Ahh, gotcha. Thanks a lot for clearing that up.