# Coefficient of Friction

1. ### panvietboy

4
Hi im having problem with this homework problem dealing with the coefficient of friction.

A hockey puck having a mass of 0.3kg is slid across the ice, starting with a speed of 12 m/s and slowing to 11.3 m/s after traveling for 3 sec. What is the coefficient of friction between the puck and the ice?

I dont even know where to start, can you guys help me please?

2. ### enricfemi

192
first,you must know the formula: F=uN

you can find out the puck N with its mass;

and find out its acceleration,

so can you afford it now?

3. ### panvietboy

4
wait im still kind of slow on this whats the first thing you should do?

### Staff: Mentor

Starting with a speed of 12 m/s and slowing to 11.3 m/s after traveling for 3 sec, determine the deceleration. Assume it is constant.

The puck has mass m=0.3kg, which experiences a downward force (weight) due to gravity on the ice of mg, where g is the acceleration due to gravity, 9.81 m/s2.

Friction is proportional to the weight, and the proportionality constant is mu, the coefficient of friction. Refer to enricfemi's post.

Also, please review one's class notes and textbook.

This is also a good reference -

http://hyperphysics.phy-astr.gsu.edu/hbase/N2st.html

Eventually one should prove a = mu * g, this type of problem.

Last edited: Mar 12, 2008
5. ### tiny-tim

26,053
Hi panvietboy! Welcome to PF!

Use Newton's second law:
force = rate of change of momentum.

What is the momentum? How much did it change? How long did it take to change?

That gives you the force.

Do you know how to work out the coefficient of friction then, from the force and the mass?

6. ### panvietboy

4
so first I would find the normal force which in this case is the mass times gravity.
N=mg---> N=(0.3kg)(9.81) = 2.943 then after I get this what do i do next

7. ### tiny-tim

26,053
Then you have to find the friction force, for which you ask yourself:

8. ### panvietboy

4
im still having trouble figuring out how to find the force. I dont get the equation I should use.

9. ### tiny-tim

26,053
Do it one step at a time, and show us your answers …

What is the intial momentum?

What is the final momentum?

10. ### PaintballerCA

25
Think about it step by step. You know the final and initial velocity right? You know how long it took to get from the initial velocity to the final velocity, so what equation do you think you need to use? What do you need to solve for? How does this relate to the force of friction? What is a force defined as?

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