Question

A person lifts a 4.5 kg cement block a vertical distance of 1.2 m and then carries the block horizontal- ly a distance of 7.3 m. determine the work done by the person and by the force of gravity in this process.

Answer 1

When a person lifts the block, the block has more potential energy. Therefore the person does positive work on the block. work = m g h work = (4.5 kg) (9.80 m/s^2) (1.2 m) work = 52.92 joules The person's work on the block is 52.92 joules When the block is being raised, the force of gravity opposes the motion. Therefore the force of gravity does negative work on the block. work = - (force) (h) work = - m g h work = -(4.5 kg) (9.80 m/s^2) (1.2 m) work = -52.92 joules The work done by the force of gravity on the block is -52.92 joules Note that when the block is moved horizontally, the potential energy does not change. Therefore there is no work done on the block when it moves horizontally (we are assuming that the kinetic energy does not change).