You keep a chalkboard eraser pressed against the chalkboard by using your finger

Question

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?

Static friction

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 eraser falls. What happens to the magnitude of force F now?

Normal force Kinetic friction Gravitational force

Static friction

B. What is the magnitude of force A?

It is equal to the gravitational force Earth exerts on the eraser. It is twice the gravitational force Earth exerts on the eraser. It is equal to the force you press the eraser against the chalkboard. It is twice the force you press the eraser against the chalkboard. It is a half the gravitational force Earth exerts on the eraser.

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

-No -Yes

D. . Now release your finger so that the eraser falls. What happens to the magnitude of force F now?

Explanation / Answer

(A)

An object remains in rest if the net force acting on that in any direction should be zero. In the direction of applied force, the Normal force by the chalkboard balances the applied force. In the direction of downward gravitational force on the chalk the force A should be balance to keep the eraser from falling. In this case the force A should be static friction force and the direction of this force will be in vertical upward direction.

(B)

From the above discussion, the force A should be balanced by the downward gravitational force. Thus, the statement the It is equal to the gravitational force Earth exerts on the eraser is correct.

(C)

No, as force A is equal to gravitational force, the force A does not depend on the amount pressing force.

(D)

After removing the finger, the force A becomes zero as the normal force is equal to zero. Since, the force A is equal to normal force multiplied by the coefficient of static friction between the chalk and board.

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