Motions and constant acceleration

Question

Which of these motions is not at constant acceleration: a rock falling from a cliff, an elevator moving from the second floor to the fifth floor making stops along the way, a dish resting on a table? Explain your answers.

Answer 1

A rock falling from a cliff is subject to the acceleration due to gravity which, on earth, is a constant value of 9.8 m/s².

An elevator moving from the second floor to the fifth floor making stops along the way must slow down to a stop (decelerate), begin moving from a stop to a certain speed (accelerate), and repeat that process of deceleration and acceleration. Consequently it does not have a CONSTANT acceleration.

A dish resting on a table has an acceleration of zero that doesn't change so, you could consider this as a constant acceleration (of zero).