7-62. 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.