Problem 7-62 Energy cons. with friction - C

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.

FBD with all forces and directions indicated.


Friction is acting so some mechanical energy is lost as thermal energy.

In this problem:

(A)   Potential energy changes.

(B)   Kinetic energy changes.

(C)   Both potential and kinetic energies change.