# Need help with a Force problem involving friction.

A student wants to determine the coefficients of static friction and kinetic friction between a box and a plank. She places the box on the plank and gradually raises one end of the plank. When the angle of inclination with the horizontal reaches 36 degrees, the box starts to slip, and it then slides 2.2 m down the plank in 1.5 s at constant acceleration. What are (a) the coefficient of static friction and (b) the coefficient of kinetic friction between the box and the plank?

I've solved part a for 0.73 as the static friction coefficient, but cannot get b using the formulas the student guide provided. What am I doing wrong?

It suggests using:

mg(sin(theta)) - f = ma

f = ukmgcos(theta)

a = 2d/t^2

Where uk is the variable for the coefficient of kinetic friction. However how can I apply these formulas if the problem does not provide the mass? I tried making the mass negligible (Setting m to 1) but that doesn't produce the right answer. What am I doing wrong?