How much work is done on the box by person pulling?

[Bones]

Homework Statement

In the figure below,

http://www.webassign.net/userimages/jshemwell@lincolnpark.il/Net%20Force/Pulling_at_angle.gif

the mass is 4.00 kg, and = 30 degrees. The surface does have friction, with s = .45 and k = .39.

If a person takes hold of the rope and, pulling at this angle with a force of T = 29 N, drags the box 3.3 meters...

a. How much work is done on the box by person pulling?

b. How much work is done on the box by the normal force?

c. How much work is done on the box by the force of friction?

d. What is the net work done on the box?

e. If the block starts from rest, what will be its velocity just as the person finishes pulling?

Homework Equations

The Attempt at a Solution

a) 29N*3.3m*cos30 = 82.9 J
b) 0
c) I took 4kg*9.8m/s^2*0.39*3.3m*cos180 = -50.45 which is not correct. What am I doing wrong??

 

[Bones]

I can figure out the rest, I just need to know what I am doing wrong in part c. I am stuck on part c.

 

[LowlyPion]

c. How much work is done on the box by the force of friction?

The force of friction over the distance.

W = F * d = u*(m*g - T*sinθ ) * d

 

[Bones]

Thanks again!