What is the coefficent of the static friction between the backpack and the table?

Question

A backpack full of blocks weighing 60.0N rests on a table in a physics laboratory classroom. A spring with a force constant of 160N/m is attached to the backpack and pulled horizontally. If the backpack begins to slide when the spring (k=160N/m) strectches by 2.40cm, what is the coefficent of the static friction between the backpack and the table?

Answer 1

Find the friction force f = kx, then compute μs = f/normal force = f/mg.