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 the table? Explain your answers.

Short answer

An elevator moving from the second floor to the fifth floor and making stops along the way does not have a constant acceleration.

Step-by-step solution

Step 1. Definition of acceleration

Acceleration is a vector quantity as it specifies both direction and magnitude. It is defined as the rate of change of velocity with respect to time.

Step 2. Examining the acceleration of a rock falling from the cliff

The motion of a rock from the cliff is free fall. If the air resistance is neglected, the rock moves under the effect of gravity and accelerates with a constant acceleration of $9.8\mathrm{m}/{\mathrm{s}}^2$.

Step 3. Examining the acceleration of an elevator moving from the second floor to the fifth floor while making stops along the way

When the elevator makes stops while moving from the second to the fifth floor, it first accelerates from rest and reaches a certain speed. To stop at the fifth floor, it decelerates. Therefore, the acceleration of the elevator is not constant. Its magnitude and direction change during the starts and stops.

Step 4. Examining the acceleration of a dish resting on the table

According to Newton’s first law, an object at rest has zero velocity. A dish kept on a table remains in its state of motion, i.e., at rest. The dish has zero velocity and hence constant acceleration (zero).