# Elevator acceleration with mass

tennisman03110
Woman stand on a scale in a moving elevator. Her mas is 60 kg, and combined mass of elevator and scale is additional 815 kg. From rest, elevator accelerates upwards. During Acceleration, the cable applies a force of 9410N. What does the scale read during acceleration. Formula or solution would be appreciated. Thanks.

## Answers and Replies

Staff Emeritus
Gold Member
Show how you started, please.

tennisman03110
Well I know F=mass*acceleration. So 9410= 875*A. A=10.754. I don't really know what the weight of the scale would read though at that time.

Staff Emeritus
Gold Member
tennisman03110 said:
Well I know F=mass*acceleration. So 9410= 875*A. A=10.754.

You need to avoid the urge to plug numbers into equations without thinking about what it is that you are doing. Newton's second law says that the net force on an object is equal to the product of its mass and acceleration. The force exerted by the cable is not the net force on the elevator. You were given 3 weights, and you need to include them in your analysis.

You should start every single one of these problems by drawing a free body diagram, and then by applying Newton's 2nd law.

I don't really know what the weight of the scale would read though at that time.

Hint: If you're standing on a scale, it doesn't read your weight, but rather the normal force applied to you.

tennisman03110
I don't really understand what the normal force would be. IS it 9410. If so then the weight would equal 9410 with no acceleration. I can't really figure out what to do to find this though, becasue their is acceleration and a lady on it.

Staff Emeritus
Gold Member
You need to analyze the problem in two parts. First, analyze the elevator and then analyze the lady on the scale.

In the first part you will get the acceleration of the elevator. You'll use that in the second part because the acceleration of the lady is the same as the acceleration of the elevator (they move together).

And again, for each part, draw a free body diagram and apply Newton's 2nd law.