Homework Help: Finding Fnet with missing variables

1. Oct 16, 2007

CharmedForever

1. The problem statement, all variables and given/known data

The eight figures below depict eight model rockets that have just had their engines shut off. All of the rockets are aimed straight up, but their speeds differ. All of the rockets are the same size and shape, but they carry different loads, so their massed differ. The specific mass and speed for each rocket is given in each figure. (In this situation, we are going to ignore any effect air resistence may have on the rockets.) At that instant when the engines are turned off, the rockets are all at the same height.

Rocket A: 30m/s and 700g
Rocket B: 40 m/s and 500g
Rocket C: 20m/s and 600g
Rocket D: 20m/s and 700g
Rocket E: 30m/s and 400g
Rocket F: 40m/s and 600g
G: 30m/s and 600g
H: 30m/s and 500g

2. Relevant equations

Fnet= ma

3. The attempt at a solution

The net force for all of them is zero based on the equation Fnet = ma. Because we were told to ignore air resistence, we assume that acceleration is zero for all the rockets and therefore the rockets are travelling at constant velocities. If we input any of the masses into the equation above, the net force for any of the rockets will be zero because the acceleration is zero. Anything times zero is always zero.

*(I desperately need help with this question. My teacher insists that there is way to find the Fnet without time but I don't see how that is possible. Any help would be appreciated. Thanx)*

Last edited: Oct 16, 2007
2. Oct 16, 2007

Woodpecker

Is this on earth or in a void? Gravity could be your key.

3. Oct 16, 2007

CharmedForever

It is on earth... How would I use gravity to answer the question... I am really lost here.

Last edited: Oct 16, 2007
4. Oct 16, 2007

Woodpecker

Then I assume your teacher expects you to use acceleration due to gravity. I'm just a student like you, so take what I say with a grain of salt, but I'm almost certain that's what he expects you to do.

5. Oct 16, 2007

CharmedForever

Alright, thanx so much... I shall try that.