Elevator Problem: Determining Scale Reading for a Moving Object

[Masrat_A]

Homework Statement

A 100kg man stands on a scale in an elevator moving downward. If the elevator decelerates at $2 m/s^2$, determine the scale reading (in kg).

Homework Equations

$N = -WT = -Mg$

The Attempt at a Solution

$-WT = -100(g)$
$-WT = -100(-10)$
$-WT = 1000$

$Ma = 100(-2)$
$Ma = -200$

$N = -WT + Ma$
$N = 1000 - 200$
$N = 800$

$kg = N/g$
$kg = 800/10$
$kg = 80$

Could any of us please check if this seems correct? Would there be any other possible ways of achieving the answer?

 

[mfb]

If the elevator is moving downwards and decelerating (getting slower), do you expect the scale reading to increase or decrease?

 

[Masrat_A]

If the elevator is moving downwards and decelerating (getting slower), do you expect the scale reading to increase or decrease?

I would expect the scale reading to decrease.

Would $N = Mg - Ma$ be a better formula to use, therefore?

$Mg = 100(10) = 1000$
$N = 1000 - 200 = 800$
$N/g = 800/10 = 80$

 

[mfb]

I would expect the scale reading to decrease.

Jump up. When you land back on Earth and slow down from the fall, do you legs have to work harder or less hard?

 

[Masrat_A]

Jump up. When you land back on Earth and slow down from the fall, do you legs have to work harder or less hard?

The legs will have to work harder when we land back on Earth.

 

[mfb]

Right. Correspondingly, a scale would read more.