An elevator car carries two identical solid wooden crates
PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
An elevator car carries two identical solid wooden crates, each of mass 25kg. One crate rests on the elevator floor, and the second crate is stacked directly on top of the first. The elevator cable pulls upward so that the car accelerates upward at constant magnitude a=2.0ms−2.
Respond to the following prompts. Assume all contact surfaces are perfectly rough enough to prevent any slipping between the crates.
(a) For the instant described, draw a clearly labeled free-body diagram for
i. the upper crate, and
ii. the lower crate.
Label each force with its type (e.g., "normal," "weight") and the object that exerts it.
(b) Using your diagrams and appropriate reasoning, explain whether the magnitude of the normal force exerted by the lower crate on the upper crate is greater than, less than, or equal to the weight of the upper crate. Support your answer with Newton’s laws.
(c) Compare the magnitudes of
i. the normal force between the two crates and
ii. the normal force exerted by the elevator floor on the lower crate
for the following three cases, explaining your reasoning in each case:
• the elevator accelerates upward at 2.0ms−2 (the situation in part (a));
• the elevator moves upward at constant velocity;
• the elevator cable suddenly snaps and the system is in free fall.
(a) For the instant described, draw a clearly labeled free-body diagram for
i. the upper crate, and
ii. the lower crate.
Label each force with its type (e.g., "normal," "weight") and the object that exerts it.
(b) Using your diagrams and appropriate reasoning, explain whether the magnitude of the normal force exerted by the lower crate on the upper crate is greater than, less than, or equal to the weight of the upper crate. Support your answer with Newton’s laws.
(c) Compare the magnitudes of
i. the normal force between the two crates and
ii. the normal force exerted by the elevator floor on the lower crate
for the following three cases, explaining your reasoning in each case:
• the elevator accelerates upward at 2.0ms−2 (the situation in part (a));
• the elevator moves upward at constant velocity;
• the elevator cable suddenly snaps and the system is in free fall.