# Homework Help: A sled is being pulled accross horizontal, snow covered ground

1. Oct 16, 2012

### smaan

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

A 16-kg sled is being pulled along the horizontal snow-covered
ground by a horizontal force of 24 N. Starting from rest, the sled attains
a speed of 2.0 m/s in 8.0 m. Find the coefficient of kinetic friction
between the runners of the sled and the snow.

2. Relevant equations

3. The attempt at a solution

I have found the acceleration of the sled by using the v^2 = v0+2ax. So, 2^2 = 2(a)(8), which gives me 0.25 m/s^2. I also calculated the normal force. Since it is a horizontal plane, it would be (9.81)(16) = 156.96 N.

Not to sure how to get the μFk from here.

2. Oct 16, 2012

### Staff: Mentor

How might you find the friction force acting on the sled? (Hint: Newton's 2nd law.)

3. Oct 16, 2012

### PeterO

If that is indeed the acceleration [I don't doubt you - I have just not checked your answer] you should know the net force, and thus how large the friction that opposes the applied 24N force.
Once you have the size of the friction force, you should be able to calculate the coefficient of friction, since you have the normal force.

4. Oct 16, 2012

### smaan

Would the frictional force be F=ma? So would F=(16kg)(0.25m/s^2)=4N?

So then, μk=Fk/FN. So, since I found the normal force to be 156.96 N, the kinetic friction coefficient would be 4/156.96 = 0.022548?

5. Oct 17, 2012

### PeterO

Newtons second law, using the actual acceleration, gives you the net Force acting.
The applied Force plus the Friction Force [adding as vectors of course] give the net Force.

The answer you give here is not correct.

6. Oct 17, 2012

### Staff: Mentor

No. When you use F = ma, remember that F is the net force. So it's better to write it as: ƩF = ma.

There are two horizontal forces acting: The pulling force and the friction force. Set up an equation and solve for the friction force.

Once you solve for the correct friction force, then this method will give you μk.