In which one of the following circumstances could mechanical energy not possibly be conserved, even if friction and air resistance are absent?

A car moves down a hill, its velocity continually increasing alongthe way.

A car moves up a hill at a constant velocity.

A car moves along level ground at a constant velocity.

A car moves up a hill, its velocity continually decreasing alongthe way.

A car moves up a hill at a constant velocity

Explanation:

Since the velocity is constant, the speed is also constant and so is the kinetic energy. However, total mechanical energy is sum of gravitational potential energy and kinetic energy, and the car is moving up the hill so its potential energy rises.

Thus, in the circumstances described the mechanical energy cannot be conserved.

The correct answer is A car moving up the hill with constant velocity.