15. [lpt] Suppose you stand on a spring scale in an elevator, in six different s

Question

15. [lpt] Suppose you stand on a spring scale in an elevator, in six different situations as listed below. Let the reading of the scale in situation n be S. Rank in order, from largest to smallest, the six scale readings. E.g., if Sf>SASB> S, enter FACDBE. If equal, enter in alphabetical order. 0 a U A)l m/s down, steady speed B) v 3 m/s up, accelerating at a ms/sup, accelerating at a 1 m/s E) vl m/s down, accelerating at a-1 m/s F) m/s m/s down, accelerating at a-9.8 D) -1 m/s down, slowing at a 1 m/s

Explanation / Answer

A)
Since the speed is steady, acceleration of the elevator is zero.
Normal force, Fn = Reading on the scale = mg
= mg

B)
ma = - mg + Fn
ma and Fn are upward and positive, mg is downward and negative.
Fn = ma + mg

C)
This is similar to the previous part. Force depends only on the change in velocity.
Fn = ma + mg

D)
ma = - mg + Fn
Here ma is positive because net force must be upward to slow down a descending mass.
Fn = ma + mg

E)
- ma = - mg + Fn
Here ma is negative since the acceleration of the elevator is downward
Fn = mg - ma

F)
This is similar to the previous case (E)
Fn = mg - ma
But here a = 9.8
Fn = m x 9.8 - m x 9.8
= 0

So the order is, BCDAEF

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