You keep a chalkboard eraser pressed against the chalkboard by using your finger to exert a horizontal force on the back of the eraser.

(a) Which type of force (call it force A) keeps the eraser from falling?

(b) What is the magnitude of force A?

(c) Does the magnitude of force A (which keeps the eraser from falling) depend on how hard you press?

(d) Now release your finger so that the craser falls. What happens to the magnitude of force A now?