# Homework Help: Find the acceleration of the elevator.

1. Dec 8, 2003

### badboyben03

how can i find the answer w/o knowing the mass?

A student stands on a bathroom scale in an elevator at rest on the 64th floor of a building. The scale reads 818 N. As the elevator moves up, the scale reading increases to 950 N, then decreases back to 818 N. Find the acceleration of the elevator.

2. Dec 8, 2003

### redrogue

Actually you can solve for the mass of the person when he is at rest. At rest, acceleration is due to gravity so:

$$F_{1}=mg$$
and
$$m=\frac{F_{1}}{g}$$

where $$g=9.81\frac{m}{s^2}$$ and $$F_{1}=818N$$.

3. Dec 8, 2003

### badboyben03

after i find the mass of the person? wut do i have to do?

4. Dec 8, 2003

### redrogue

950N is the normal force the person experiences.

The person's weight must be subtracted out to get the net force which is 950N-818N=132N.

Using Newton's 2nd Law, we can can find the acceleration.

$$F=ma$$

implies

$$a=\frac{F}{m}$$

where F =132N and m is the mass of the person found.

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