# Homework Help: Problem about finding the work done by friction force

1. Sep 16, 2011

### sample5

1. The problem statement, all variables and given/known data

A block of mass M=3.4 kg is dragged over a horizontal surface by a force F=20.0 N. The block is displaced a distance d=17.0 m, the friction force Ff is 3.3 N. What is the work done by the friction force?

2. Relevant equations

Ff=μkFn

3. The attempt at a solution

I solved a problem before this one in finding the applied force but this one is very confusing to me. I have no idea where to even start. Thank you!

2. Sep 16, 2011

### himanshu07

Work done by friction is always negative. The work done by a constant force is always the dot product of the force multiplied the displacement whether or not the force is the cause of the displacement. So the work done would be -3.3*17 = -56.1 Joules.

3. Sep 16, 2011

### danielakkerma

Why don't we start with recalling how to calculate the work done by any Force?
$\Large W = \int \vec{F} \cdot d\vec{r}$
We can reduce this to simply:
$\vec{F}\cdot{\vec{x}}$
Now the rest is pretty straightforward in your case.
Hope that takes care of the confusion,
Daniel

4. Sep 16, 2011

### sample5

Thank you so much!!!