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