# Homework Help: Coefficients of kinetic friction, help PLEASE!

1. Sep 1, 2007

### myelevatorbeat

Here is the problem:

A hockey puck is hit on a frozen lake and starts moving with a speed of 13.0 m/s. Five seconds later, its speed is 5.20 m/s.

(a) What is its average acceleration?

I used the kinematic equation V=Vo+at to find the accerlation which came out to be -1.56 m/s^2 (this answer is right).

(b) What is the average value of the coefficient of kinetic friction between puck and ice?

(c) How far does the puck travel during the 5.00 s interval? m

I used X-Xo=1/2[V+Vo]t to solve this and got 45.5 m, which is right.

So basically, I'm stuck on part B of this problem. I don't remember my professor going over how to solve this particular type of problem in class so I was hoping someone here could get me started. I'm not asking for the answer, just an explanation of how to solve Part B.

2. Sep 1, 2007

### hage567

Think about forces and Newton's second law. You have already figured out the acceleration. What's the definition of kinetic friction?

3. Sep 1, 2007

### myelevatorbeat

I assume I would make F=ma equal to Fk=uN.

However, I don't see how I can do this since the mass wasn't given.

4. Sep 1, 2007

### hage567

OK, but what is the normal force equal to?

5. Sep 1, 2007

### myelevatorbeat

The normal force is equal to the weight, which is mass x gravity, but again I get lost because I don't have the mass.

6. Sep 1, 2007

### myelevatorbeat

Wait, nevermind. I got it. I divided acceleration/gravity and came out with the answer.