Teaching Physics with the Physics Suite

Edward F. Redish

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Riding an elevator

A passenger is standing on a scale in an elevator. The building has a height of 500 feet, the passenger has a mass of 80 kg, and the scale has a mass of 7 kg. The scale sits on the floor of the elevator. You may take g = 10 N/kg.

(a) Draw free-body diagrams for the passenger and the scale while the scale is sitting at rest on the 33rd floor. Somewhere in your diagram or labeling be sure to identify: (1) the type of force, (2) the object causing the force, and (3) the object feeling the force. Indicate which (if any) two individual forces in these diagrams have the same magnitude.

(b) The elevator now begins to descend. Starting from rest, it takes the elevator 6 seconds to get up to its downward speed of 8 m/s. Assuming that it is accelerating downward at a uniform rate during these 6 seconds, which of the forces in your diagram for (a) will change? For each force that changes, specify whether it will become bigger or smaller.

(c) While it is accelerating downward, which of the forces in your diagrams have the same magnitude? For each equality you claim, explain why you think they are equal.

(d) While it is accelerating downward, what does the scale read?

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Page last modified September 6, 2004