1) You are standing on a bathroom scale inside an elevator. The scale reads 95%
ID: 1299357 • Letter: 1
Question
1) You are standing on a bathroom scale inside an elevator. The scale reads 95% of your weight. What statement is correct?
A) you could be slowing down while moving upward
B) none of the other statements is correct
C) you must be at rest
D) you are moving upward at a constant velocity
E) you could be slowing while moving upward
2) You are standing on a bathroom scale inside an elevator. The scale reads 7% more than your weight. Which statement is correct?
A) None of the other statements is correct
B) You must be at rest
C) You could be speeding, while moving upwards
D) You could be slowing down, while moving upward
E) You are moving downward at a constant velocity
Explanation / Answer
1) you are moving upward at a constant velocity
In this case, the elevator and the person are starting from rest at a lower
floor. The elevator accelerates upward. The inertia of the person would
prefer to stay stationary, so the elevator floor and scale must push up on
the person to accelerate him upward along with the elevator. (The person
doesn't sink into the floor when the elevator accelerates up. The elevator
and the scale and the person all move together.)
The scale therefore has to push upward with extra force on the person to
accelerate the person's mass upward. This results in a greater contact
force between the scale and the person. Therefore the Normal Force is
larger, so the reading on the scale is a number that is GREATER than the
true weight.
2) You are moving downward at a constant velocity
In this case, the elevator and the person are initially moving upward at a
constant speed and slowing down to rest at a higher floor. The
acceleration of the elevator is downward (opposite to the upward motion,
which causes a reduction of the velocity). The inertia of the person
would prefer to keep moving upward at a constant speed, so the elevator
floor and scale effectively drop out a little bit from underneath the person
as the elevator slows down.
The person doesn't float upward, because again the elevator and the
person move together, but the contact force between the person and the
scale is reduced. The scale therefore has to push upward with less force
on the person to support the person's weight. Therefore the Normal Force is smaller, so the
reading on the scale is a number that is LESS than the true weight
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