# Homework Help: Forces problem.

1. Dec 20, 2007

### pebbles

1. The problem statement, all variables and given/known data
Joe puts 11400 J of energy to drag crate 25 m across floor with constant speed. Rope makes 48 degree angle with horizontal.
Force exerted by the rope on the crate?
What is the force of friction acting on the crate to impede its motion?
What work is done by the floor through the force of friction between floor and the crate?

2. Relevant equations
See #3.

3. The attempt at a solution
a. I thought I would need to calculate the horizontal component with the 25 m and 48 degree angle and the cosine function, but got the wrong answer.
b. Ffriction=coefficient of kinetic frictionmg. But I don't know mass, so I'm not sure what equation I'd use.
c. W=Fd...Need to correctly calculate part b.

I'd appreciate any help. I'm not so good at physics, so it'd help if you really explain.

Thanks.

2. Dec 21, 2007

### mezarashi

3a: Can you explain more clearly what exactly you're doing with the horizontal component and the 25m and the 48 degree angle and the cosine function? The hint in the question here is that he's done 11.4kJ of work. Work can only be done where there is displacement (i.e. in the x-direction).

3b-c: I'd consider these somewhat "trick" questions for the answers are not hard to get if you have a good conceptual understanding of work. What conditions in the x-direction must be satisfied for there to be constant velocity (no acceleration)? What does the free body diagram in the x-direction look like?