A hand lifts a block vertically upward at constant velocity. The work done by gravity on the block if the system consists of the block only. The work done by gravity on the block if the system consists of the block and the earth. AND WHY?

1. is zero, is zero2. is zero, is negative3. is zero, is positive4. is negative, is negative5. is positive, is positive

If the system consists of the block only, the work done by the gravity is negative.

If the system consists of the block and the earth the work done by the gravity is zero.

Explanation:

If the system consists of the block only, then the system experiences two external forces: one exerted by the hand that lifts the block vertically upward and other exerted by the earth (gravity), which is opposed to the movement of the system, so the work done by gravity is negative.

On the other hand, if the system consists of the block and the earth, then only exists a external force which is the exerted by the hand. So, the force exerted by gravity is zero.

the motion of a pendulum is a classic example of mechanical energy conservation. a pendulum consists of a mass (known as a bob ) attached by a string to a pivot point. as the pendulum moves it sweeps out a circular arc, moving back and forth in a periodic fashion.

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