# Homework Help: How to find the coefficient of Friction?

1. Apr 11, 2013

### luxford

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

A small 15 kg cardboard box is thrown across a level floor. It slides a distance of 7.0 m, stopping in 3.3 s. What is the coefficient of friction between the box and the floor?

2. Relevant equations

3. The attempt at a solution

I didn't know how to do it. I just drew a free-body diagram of it. :|

2. Apr 11, 2013

### haruspex

What forces do you have in your FBD? What is the deceleration (a) according to your forces equation and (b) according to the given data?

3. Apr 11, 2013

### luxford

The forces I have on my FBD is the force of gravity, normal force, and force of friction.

As for deceleration, it is not given...

4. Apr 11, 2013

### haruspex

You know the equation ƩF=ma, right? Apply this in the horizontal and vertical directions to obtain two equations.

5. Apr 11, 2013

### rude man

What's the average speed? So then what's the initial speed?
Then, can we equate initial k.e. with friction energy loss to compute the friction coefficient?

6. Apr 11, 2013

### Dewgale

If you have t, d, and vf, you can find the deceleration using kinematics.

Since you have all three of these, finding deceleration is quite simple.

d=vft+$\frac{1}{2}$at2