# Homework Help: A Force Question

1. May 12, 2008

### inflames829

1. The problem statement, all variables and given/known data
A force F=-4.3i+2.0j+1.5k Newtons exerts a displacement of d=-0.8i+1.8j+-3.6k metres upon a small (ie mass is negligible) object. If the friction force is equal to Ffr=-0.2,, then what is the net work done upon the object?

2. Relevant equations

3. The attempt at a solution
I know the net work is the sum of all the individual work and the work equation is W=F.D. So i tried to work out the work of the initial force stated by multiplying the force and displacement stated which i got 1.64J. I think the only other work is caused by the friction force but when i try to work that out (because the mass is negligable) i get 0. Can someone please help?

2. May 12, 2008

### Tedjn

Mass is negligible only affects calculating the friction force using $\mu N$, but you already have the friction force. What can you do with it?

3. May 12, 2008

### inflames829

could i put it as the force in the work equation and put the distance as the displacement thats stated in the question?

4. May 12, 2008

### Phlogistonian

Yes.

5. May 12, 2008

### inflames829

so i have 1.64 from the first force i worked out and if i put the friction force into the work equation w= -0.2 x D i get -0.82. If I add the two works i have i get 0.82, but the answer is 1.3.... can you give me hints on what ive done wrong??

6. May 12, 2008

### Tedjn

I get that also. From where did you get this problem?