- #1
Kirasagi
- 9
- 0
Homework Statement
When the elevator is at rest, the scale reads W.
Suppose the elevator now accelerates down-ward at a constant rate of 0.4 g.
What is the ratio of the new scale reading to the value W of the scale reading when the
elevator is at rest?
Homework Equations
Force equations:
F = ma
Fnet = 0
The Attempt at a Solution
What I did is to draw a force diagram that resulted:
At rest:
Fn - mg = 0
mg = Fn
Accelerating:
Fn - mg = -ma, where a = .4g
mg = Fn + .4mg
I set Fn = mg = 1, so ratio becomes to 1.4 -> (1 + .4)/(1)
The real answer is .6, but I'm not really sure how to get that. I know if I do:
Fn - mg = ma
mg = Fn - .4mg, the ratio then becomes to .6
Why is it "ma" instead of "-ma" when the elevator is accelerating downwards?