# Homework Help: Work QUICK: And Energy Problem Involving Air Resistance

1. Oct 5, 2006

### cheechnchong

QUICK: Work And Energy Problem Involving Air Resistance

a projectile mass of 0.750 kg is shot straight up with an intial speed of 18.0 m/s. (a) how high would it go if there were no friction? (b) if the projectile rises to a max height of only 11.8 m, determine the magnitude of the avg force due to air resistance.

Im confused about the 2nd part..the 1st part isn't a prob (i got hf=16.5 m)

Last edited: Oct 5, 2006
2. Oct 5, 2006

### arildno

Use the work-energy theorem in both cases.

3. Oct 5, 2006

### cheechnchong

how does that work?

KE(initial) + PE(initial) = KE(final) + PE(final) ? like this...i get for part A--16.5m

now if i work on the 2nd part...i dont even know how to calculate the air resistance (which is a force)! where does the force come from the equation above? thats where im stuck right now...(haha this is due in an hour ouch)

Last edited: Oct 5, 2006
4. Oct 5, 2006

### arildno

No, that is the principle of conservation of mechanical energy when there are no non-conservative forces acting upon the system. In b) you DO have a non-conservative force!

5. Oct 5, 2006

### cheechnchong

WOW, what an assignment then...the prof didn't even touch the subject of non-conservative forces--although as i look ahead in the chapt it's there! let me see what i can pick up from it and i'll post back with another question..thanks arildno

so the way i solved the first part wrong?

6. Oct 5, 2006

### arildno

Well, you HAVE learnt of work, have you?

Compute the mechanical energy loss in b), then calculate the average force from air resistance against which work has been done by the system.

7. Oct 5, 2006

### cheechnchong

ok, i had you when you said compute the mechanical energy loss...but how am i supposed to calculate avg force from air resistance agains work done by system? which equation do i use (or do reuse)?

8. Oct 5, 2006

### arildno

well, work is FORCE TIMES DISTANCE, right?

9. Oct 5, 2006

### cheechnchong

yesireee hahah...ok, then im makin it harder on myself! thanks, thanks...