# Homework Help: Forces help!

1. Sep 30, 2007

### Lorelyn

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

Three forces act on a particle that moves with a constant velocity v = 2 m/s x – 7 m/s y . Two of the forces acting on this particle are F1 = -10.0N x + 9.0N y + 4.0N z and F2 = = -1.0N x + 4.0N y + 2.0N z . What is the magnitude of the third force?

2. Relevant equations

Fnet=ma?

3. The attempt at a solution

I'm actually not even sure what the question is asking to be honest....

Last edited: Sep 30, 2007
2. Sep 30, 2007

### Dick

F=ma, indeed. So a=0 since it's moving with constant velocity, so the sum of the forces is zero. What's the third force? Once you have it, figure out it's magnitude.

3. Sep 30, 2007

### Lorelyn

So I just go:

Fx: -10-1+____=2
Fy: 9+4+___=-7
Fz: 4+2+__=0
Where the blanks are the third force?

4. Sep 30, 2007

### Dick

No. The velocity is CONSTANT. So the acceleration is ZERO. Set all of your equations equal to zero.

5. Oct 4, 2007

### Lorelyn

All of which equations? I get that Fnet = 0 because there is no acceleration, but how do I relate that to velocity? or to the components of the other forces?

6. Oct 4, 2007

### Dick

All of THESE equations. The velocity doesn't matter since it's constant. The equations above give you the components of Fnet. Which should be zero, so replace the 2 and -7 with 0. Now figure out what goes in the blanks to give you the third force.