physics ( disregard last post)

hey bob,
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.
does it matter how much i stretch it and what does it mean when
"You can measure k pretty easily (force=kx, use a spring scale to measure the force at some x)"
i already have k which is a mass using a balance beam. Ek=1/2 mass v^2.what do u mean some x. how i possibly measure the mass at a particular velocity. and can i use any velocity.

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.

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