# Box problem on work/force/energy

1. Feb 2, 2014

### brnnpink4

1. The problem statement, all variables and given/known data
A 50Kg box is pulled across the ground with a force of 250 N. The speed of the box is a constant 4m/s for 3 seconds. Friction is NOT 0.
a) What is the net work done on the box by all forces ?
b) How much work is done on the box by applied forces?
c) What is the work done by the friction on the box?
d) What is the power output of the applied force?
e) If there were no friction, and the force was applied for 8m, what would the resulting speed be if the box started from rest?

2. Relevant equations
f=ma
KE=.5mv^2
PE=mgh
P=fvcosx

3. The attempt at a solution
b) w=250d
c)w=frkd (Friction constant and distance are what I cant find)
d) p=w/3
e)

I guess what is mostly stumping me on these problems is the fact that I dont know how to find the distance or the spring constant

2. Feb 2, 2014

### jackarms

Your answers to b and d look right. For a, remember that net work is net force times displacement, so you just have to find the net force acting on the box.

For c, you don't have to know the friction constant -- net work equals the sum of all works, in this case friction and applied. If you have net work and the applied work, then you can find the work done by friction.

For e, there's no friction, so the applied work will feed directly into the kinetic energy of the box, and from there you can find the final speed.