# Homework Help: Force, acceleration, gravity question

1. Oct 4, 2005

### Kristin

Hi, I've been working on my homework assignment, and there are a couple tricky questions that I hope someone can give me a few pointers on.

Question:
Compressed air is used to fire a 60 g ball vertically upward from a 5.3 m tall tube. The air exerts an upward force of 3.3 N on the ball as long as it is in the tube. How high does the ball go above the top of the tube?

I have tried a few things:
First, I tried finding the acceleration in the tube and then using that to find the speed at the end of the tube.

F=ma
a=55.0m/s/s
then vf^2=vi^2+2a*d
vf= 17.38 m/s

Then to find height, I used vf^2=vi^2+2a*d with vf from the tube as the new vi, and vf=0 because it's at the max height.
Using 0=17.38^2+2(-9.8)d
d=15.41 m (which is incorrect)

I also tried d=.5*gt^2 and got d=35m (also incorrect)

If anyone can give me a better direction, I would be very greatful,

2. Oct 4, 2005

### Staff: Mentor

To find the acceleration, first find the net force on the ball. (You forgot about gravity.)

3. Oct 4, 2005

### Kristin

That makes sense, but following the same steps using net force as applied-gravity (net force= 2.712N), I get d to be 24.4 m, which is still not the correct answer.

4. Oct 4, 2005