Coefficient of kinetic friction of force sensor

[map7s]

Homework Statement

A force sensor is used to measure a force applied to a 8.2 kg box on a hard, horizontal surface. It records that the box starts to move when the force is equal to 20.7 N. This force makes the box start to move with an acceleration of 0.4 m/s2. Find s and k respectively.

Homework Equations

f s=mu s*N
f k=mu k*N

The Attempt at a Solution

I found out what mu s was, but I am having trouble finding out mu k. I tried F=mg*mu k and ma=mg*mu k. Is there another way to go about solving this part of the problem?

 

[Doc Al]

Set the net force equal to ma. (Two horizontal forces act on the box.)

 

[map7s]

oh...okay thanks! that helped!