Question

A person is riding on a Ferris wheel of radius R. He starts at the lowest point of the wheel. When the wheel makes one complete revolution, is the net work done by the gravitational force positive, zero or negative? Do you need to know how the speed of the person changed before you can answer the question?

Answer 1

Answering the two questions in reverse order:

-- No. I don't need to know how the speed of the person changed before I can answer the question.  I can answer it now.

-- The NET work done by the gravitational force is zero.

-- As the person and his girl-friend go up the first half of the wheel, the motor does positive work and gravity does negative work.

-- After they pass the peak at the top and come down the second half of the wheel, the motor does negative work and gravity does positive work, even though the couple may be interested in other things during that time.

-- The total work done by gravity in one complete revolution is zero.

-- The total work done by the motor in one complete revolution is only what it takes to pay back the energy robbed by friction and air resistance.