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
The work done by gravity on the block is negative if the system consists of the block only. The work done by gravity on the block is zero if the system consists of the block and the earth.
According to the work-energy theorem, the total work is equal to the change in the energy of a system due to external forces
The work done by a force on a system is positive when the force and the displacement of the object is in the same direction and negative when they point in opposite directions
According to the question, the force exerted by the hand moves the block vertically upwards but the displacement of the block reduces with time (in the opposite direction) since it decelerates. So 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 the overall work done against the block and the earth is zero.
Learn more about work-energy theorem:
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