Posted by **Heather** on Thursday, November 3, 2011 at 6:17pm.

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**, Thursday, November 3, 2011 at 7:30pm
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**, Thursday, November 3, 2011 at 8:53pm
Ahh, gotcha. Thanks a lot for clearing that up.

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