Question

A large fish hangs from a spring balance supported from the roof of an elevator.

Part A

If the elevator has an upward acceleration of 2.90m/s2 and the balance reads 50.0N , what is the true weight of the fish?

Part B

Under what circumstances will the balance read 32.0N ?

Part C

What will the balance read if the elevator cable breaks?

Answer 1

Force = weight + mass x acceleration
Force = mass x g + mass x acceleration
F = m(g + a)
50.0 = m(9.81 + 2.90)
m = 50.0/(12.71) = 3.93... kg
weight = mass x g = 3.93... x 9.81 = 38.6 N

(b)
34.0 = 3.93...(9.81 + a)
a = 34.0/3.93... - 9.81 = -1.1672 m/s^2

The elevator would be accelerating downwards at 1.1672 m/s^2.

There are two ways in which a NORMAL elevator could accelerate downward. (Of course an elevator with a large fish hanging from the ceiling is not normal.)

1) The elevator could go from being stopped to starting to go down. This is an acceleration downwards.

2) The elevator could be going up and come to a stop. This is also negative acceleration. Its speed up is decreasing.
4 years ago "

For the third part of your question, if the elevator cable breaks, the emergency brakes will operate and the elevator will stop. The balance will then read the stationary weight of the fish. The elevator will NOT go into free fall.