Physics
posted by Heather .
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.

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. 
Ahh, gotcha. Thanks a lot for clearing that up.