A backpack full of books weighing 52 Newton rests on a table in a physics classroom. A spring with a force constant of 150 N/m is attached to the back pack and pulled horizontally.
(a) If the spring is pulled until it stretches 2 centimeter and the pack remains at rest, find the force of friction exerted on the backpack by table?
(b) Does the answer to part (a) change if the backpack is doubled?