A professor holds an eraser against a vertical chalkboard by pushing horizontally on it. He pushes with a force that is much greater than is required to hold the eraser. The force of friction exerted by the board on the eraser increases if he:

1.pushes so his force is slightly downward but has the same magnitude
2.pushes with slightly less force
3.stops pushing
4.pushes with slightly greater force
5.pushes so his force is slightly upward but has the same magnitude