# Homework Help: Finding the coefficient of friction

1. Oct 6, 2011

### Singdasorrow

1. The problem statement, all variables and given/known data
A 100 kg bale of hay falls off a truck traveling along a level road at 24.4 m/s. It lands flat on the road and slides 100m before coming to rest. Assume the acceleration is constant, compute the coefficeient of kinetic friction between the road and the hay.

2. Relevant equations
F=u x N
Sum of the forces horizontally (x) = 2440 N -f = ma (acceleration is a constant)
Sum of the forces Vertically (y) = N-W=O

3. The attempt at a solution
f= (2440)/(100a)
W= -980 so N= -980
now i plugged into f= u x N
(2440/100a)= u x -980
u= -.02a

Now i know the answer because i can see it in the back of the book, and this doesnt feel right. advice?

2. Oct 6, 2011

### Staff: Mentor

Don't look in the back of the book?

Can you explain where your numbers come from? What's 2440N? It would appear to have the same digits as the initial velocity but I don't see a calculation behind it.

3. Oct 6, 2011

### Singdasorrow

i made the initial velocity into a force. 24.4 m/s x 100kg =2440 N

4. Oct 6, 2011

### Staff: Mentor

When you get an answer that looks like the calibre of a bullet, you're probably right.

5. Oct 6, 2011

### Staff: Mentor

Velocity is not force. Mass x Velocity is momentum, not force. They are quite different things.

Hint: Find the acceleration given the provided information. Then use Newton's second law to find the force that acted to produce that acceleration.

6. Oct 6, 2011

### Singdasorrow

thank you so much! that made things so much easier for me <3