A person stands on a scale inside an elevator at rest (see the figure below). The scale reads W = 621 N.

Question

What is the person's mass? (kg)

The elevator accelerates upward momentarily at the rate of a = 4 m/s². What does the scale read then? (N)

 The elevator then moves with a steady speed of 7 m/s. What does the scale read now? (N)

Answer 1

W = m*g, so the mass is equal to W/g

mass = W/g= (621 N)/(9.81 m/s^2), m = 63.303 kg

When the elevator accelerates upwards, the scale has to support the person's weight and the force that comes from the acceleration of the elevator. So the scale reads:

Scale reading = m*g + m*a = (63.303 kg)*(9.81 m/s^2) + (63.303 kg)*(4 m/s^2) = 874.212 N

Once the elevator achieves a steady speed of 7 m/s upwards, the acceleration of the elevator is zero. Therefore, the reading on the scale goes back to normal, which is 621 N.