# Homework Help: Apparent weight on an elevator. Magnitude of its acceleration?

1. Oct 2, 2008

### WPCareyDevil

1. The problem statement, all variables and given/known data
You are standing on a bathroom scale inside an elevator. Your weight is 160 lb, but the reading of the scale is 123 lb.
(a) What is the magnitude and direction of the acceleration of the elevator?
??? m/s2
(b) Can you tell whether the elevator is speeding up or slowing down?
do not know
slowing down
speeding up

2. Relevant equations
weight=mg
ma=ma

3. The attempt at a solution

160lbs = 75.574kg

123lbs: 55.792kg

g= 9.8ms

obviously, the elevator is accelerating downwards, reducing his apparent weight.

his weight when on the ground is 740.625N.

So, 740.625=55.792a
a=13.27475. The question is asking for magnitude, but there is no x component so sqrt(a^2) would still be 13.27475

Obviously, this isnt correct. Can someone point me in the right direction?

2. Oct 2, 2008

### LowlyPion

If it was slowing down while going up it could read the same. All you know is acceleration is less down on the scales.

3. Oct 2, 2008

### WPCareyDevil

Ahh, you're right. That makes sense - I didnt think of that.

I tried using -13.27, and subtracting 9.8. What am I missing with the calculation?

Thank you for the tip!

4. Oct 2, 2008

### LowlyPion

You can divide by like equations.

9.8/160 = x/123

I know the units are silly but they cancel as long as your acceleration is to be in m/s2

The difference then with 9.8 must be the acceleration in the same direction as g.

5. Oct 3, 2008

### WPCareyDevil

Thank you so very much!!

I was able to figure it out after a bit of tweaking.

This place is the best =D. I've tried to go through answering peoples questions- but I find that I either cannot answer them well enough, or someone has already beat me to it. I'll try to give back as much as I can

6. Oct 3, 2008

### LowlyPion

You're welcome. Glad that it helped. All contributions welcome.

Cheers.

7. Oct 3, 2008

### Sakha

Maybe I'm wrong, but I think it should be 546.762=75.574a so a=7.234.