A small block is at rest on the floor at the front of a railroad boxcar that has length . The coefficient of kinetic friction between the floor of the car and the block is µk. The car, originally at rest, begins to move with acceleration a. The block slides back horizontally until it hits the back wall of the car.
(a) At the moment it hits, what is its speed relative to the car?
(b) At the moment it hits, what is its speed relative to the Earth?