# Find the Coefficient of Kinetic Friction

1. Sep 16, 2011

### firewareagle

I have this homework question: A sled is pulled with a horizontal force of 15 N along a level trail, and the acceleration is found to be 0.40 m/s2. An extra mass m = 4.3 kg is placed on the sled. If the same force is just barely able to keep the sled moving, what is the coefficient of kinetic friction between the sled and the trail?

Important equations:
Fk = uk *N
F = ma

Attempt to solve:
Sum of all forces = ma
sum of all forces = Fpull - Fk
Fpull - Fk = ma
Fpull - ma = Fk
Fpull - ma = uk *N
(Fpull - ma)/mg = uk

I'm unable to solve since I don't know what the original mass was, since it says 4.3 extra mass was added. Can someone help me out??

2. Sep 16, 2011

### Mordred

the force and acceleration is given to you use those values to first calculate the mass. essentially your question is two parts the first part is to calculate the original mass using F=ma use that resulatant and add your new mass then calculate the kinetic friction

Last edited: Sep 16, 2011
3. Sep 16, 2011

### firewareagle

right I did that, but i still didn't get the right answer

that way the answer would be 0.0041

but the answer is actually 0.01

4. Sep 17, 2011

### Staff: Mentor

Write two equations: One for before the 4.3 kg mass is added and one for after. You'll have two equations and two unknowns.