How Do You Calculate the Average Force of Air Resistance on a Projectile?

[neoncrazy101]

Homework Statement

A projectile of mass 0.656 kg is shot straight up with an initial speed of 25.4 m/s. (a) How high would it go if there were no air resistance? (b) If the projectile rises to a maximum height of only 7.64 m, determine the magnitude of the average force due to air resistance.

Homework Equations

1/2mv_f^2 + mgh_f = 1/2mv_o^2 + mgh_o

Work = KE+ PE
W = Fcos(90)s

The Attempt at a Solution

Ok, I am able to get the answer for A: (32.9m) but with B, I cannot get it. What should I do to get the answer for B? Whenever I put the equation into W = Fcos(90)s I come up with zero which is not the answer. So I don't know what I'm doing wrong.

 

[SteamKing]

Well, cos(90) = 0 (some sort of trig identity I suppose), so your equation for the work done by the air resistance is wrong. Looking at the problem, the projectile is shot straight up, and the air resistance acts to retard this motion. The reduction in trajectory height would be similar to using the same effort to launch a heavier projectile upward.

 

[Andrew Mason]

Ok, I am able to get the answer for A: (32.9m) but with B, I cannot get it. What should I do to get the answer for B? Whenever I put the equation into W = Fcos(90)s I come up with zero which is not the answer. So I don't know what I'm doing wrong.

What is the work done by air resistance? (hint: what is the difference in maximum potential energy between A and B?).

Now if that work is done by a constant force over the distance that the projectile moves, what is that force?

AM

 

[neoncrazy101]

The difference in PE between A and B is 49.116032.

I'm probably very very confused (most likely, brain is fried plus headache) but if I take that 49.116032 and multiply it by the 7.64m then I come with up 375.2 which is really large.

EDIT: I figured it out! W=162.50248 (roughly) and then F = W/D thus 162.50248/7.64 = 21.3ish.

 

[Andrew Mason]

The difference in PE between A and B is 49.116032.

I'm probably very very confused (most likely, brain is fried plus headache) but if I take that 49.116032 and multiply it by the 7.64m then I come with up 375.2 which is really large.

EDIT: I figured it out! W=162.50248 (roughly) and then F = W/D thus 162.50248/7.64 = 21.3ish.

How did you get this value for W? (you should show your work). What are the units of force?

AM