Determining the coefficient of kinetic friction of a sled with a mass added.

[waqaszeb]

Homework Statement

A sled is pulled with a horizontal force of 15 N along a level trail, and the acceleration is found to be 0.38 m/s2. An extra mass m = 4.2 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?

Homework Equations

F(net) = ma
F(friction) = μN

The Attempt at a Solution

I used the given information to get the mass of the sled, so that way I could later add 4.2 kg to it and get the total mass and calculate the coefficients from there.

Before we place the 4.2 kg mass on the object, F(net) = F(gravity)+N+F(friction)
$\Sigma$F = -mg + N + μN = ma (Fnet = ma)
ma=-mg+N+μN
m(a+g)=N(1+μ)

there is where i get lost, too many unknowns.

 

[frogjg2003]

F(net) = F(gravity)+N+F(friction)

You're right, except that all of these quantities are vectors. Draw a free body diagram with all the forces, and look at it component-wise.