hey bob,

Question

this question
average force of friction
thanks a lot. i only don't get one part.

Now, set the potential energy of the rubber band (stretched x cm) equal to the work done on friction

why is this equal to work done by friction?
how come we can't use velocity, acceleration method but instead deal with the transfer of energy?
thank you very much bob.

You have no way to MEASURE velocity or acceleration. The simple and direct way is with energy concepts. The energy of the rubber band is disspated in work done on friction.

Why is it equal? Where else could the energy stored in the rubber band go? All movement is stopped.