Calculating the Magnitude of a Force

[Lorelyn]

Homework Statement

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?

Homework Equations

Fnet=ma?

The Attempt at a Solution

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

 

[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.

 

[Lorelyn]

So I just go:

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

 

[Dick]

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

 

[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?

 

[Dick]

So I just go:

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

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.