# Problem 7-62 Energy cons. with friction

A pen of mass $0.057\; kg$ is sliding across a horizontal desk. In sliding $25\; cm$, its speed decreases to $5.7 \;cm/s$. What was its initial speed if the force of kinetic friction exerted on the pen by the desk is $0.15\; N$ in magnitude? Use conservation of energy.

In the motion of the pen:

(A)   Momentum is conserved.

(B)   Mechanical energy is conserved.

(C)   Total energy (including thermal) is conserved.