Q. A relaxed spring with spring constant k = 50 N/m is stretched a distance di = 56 cm and held there. A block of mass M = 5 kg is attached to the spring. The spring is then released from rest and contracts, dragging the block across a rough horizontal floor until it stops without passing through the relaxed position, at which point the spring is stretched by an amount df = di/9. What is the coefficient of kinetic friction µk between the block and the floor? Here is what I did: At the point the block stops the spring force is equal to the force of friction. Fspring = Ffriction k*x = Uk * m * g Uk = (K * x )/ m* g Uk = 0.0603, which the computer says is wrong.