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.