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

Staff: Mentor

That's the answer I get. (Your method is fine.)

5. Oct 4, 2005

Kristin

The program that I am imputing the answers into still tells me that I don't have the correct answer, so I think that it might be missing something still.

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