# An Elevator Problem

1. Oct 7, 2004

### mojo2185

Here's the problem:
A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads only 0.75 of the person's regular weight. Calculate the acceleration of the elevator, and find the direction of acceleration.

I honestly don't know how to get this problem started. I'm assuming gravity has a part to play in this, and based on the "0.75", i'm guessing the elevator is going down. Can anyone offer tips to get this problem rolling?

2. Oct 7, 2004

### Pyrrhus

Do a force analysis using

Newton's 2nd Law

$$\sum_{i=1}^{n} \vec{F}_{i} = m \vec{a}$$

3. Oct 7, 2004

### mojo2185

I'm a little rusty on my physics (i'm actually helping a friend here, its not my problem). Could you please explain a little more what you mean?

One more thing, calculus shouldn't be required for this problem, it just covers Newton's Laws (as you mentioned). I think i'm overlooking something obvious here :grumpy:

Thanks for taking a look, i really appreciate it!

Last edited: Oct 7, 2004
4. Oct 7, 2004

### Pyrrhus

The person will have a movement while in the elevator in the Y-axis. or an acceleration of $$\vec{a} = +/- a_{y}j$$, depending where its heading. Which you have to find out. Test it on both directions and see which equation is the proper one.

Last edited: Oct 7, 2004
5. Oct 7, 2004

### mojo2185

Ok so based on that (and the info i was given)... it looks like the elevator is heading downward, with the person's acceleration increasing in the $$- a_{y}j$$ direction. Am I on the right track?

Last edited: Oct 7, 2004
6. Oct 7, 2004

### Gokul43201

Staff Emeritus
Right track, so far ! But it wouldn't be right to say "acceleration increasing in the..". There is no indication that the acceleration is changing.

But yes, the acceleration is downwards.

7. Oct 7, 2004

### Gokul43201

Staff Emeritus
Did you draw a free body diagram ? This is the first thing to do. What are the forces on the man ? Relate the net force to the acceleration (see post #2) and you're home !

8. Oct 12, 2004

### mojo2185

Thanks for the help, we got it finally!