Question

A hand moves a block vertically upward. Its speed slows down as it moves up (it is decelerating). 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.

Answer 1

Answer:

a. negative

b. zero

Explanation:

work is the change in the energy of a system due to external forces

work is done when a force moves a load through a distance in the direction of the force

The work done by a force on a  system is positive when the force  and the force displacement point  in the same direction and negative  when they point in opposite  directions

in the scenario we are considering in the question, the force (the hand) moves the block vertically upwards but the displacement of the block reduces (in the opposite direction) since it decelerates. as such the work done by gravity on the block is negative.

the work done on the block is zero if the system consists of the block and the earth because there is no gravitational force exerted on the system