Question

Physics,
https://www.dropbox.com/s/qfcy5mzh2vntedi/Screenshot%202015-11-14%2002.16.30.png?dl=0

Answer 1

\[\sum_{}^{} F = -mg + F_N\]

Answer 2

so
\[F_N = mg + ma\]

Answer 3

It's important to know that the scale reads \(F_N\). A simple application of Newton's second law of motion will allow you to analyse this situation.

Answer 4

Take what ospreytriple said and apply it to the formula i posted above for (a)

Answer 5

I think the scale reads fn - mg . Am I right in assuming that ?

Answer 6

For part a, we can add up the accelerations, since they are in the same direction (save for a minus sign):

\[F = m\Sigma_i a_i\]

What we need to be careful about are those signs.

The normal force pushes upward on the woman. This is what the scale actually reads. Let's think about how to add these accelerations to find what the scale reads. If we are in free fall while standing on the scale, the scale will read a force of 0, as it's not pushing back against us, just falling with us. So, if the elevator is accelerating downward, we'd expect the force read by the scale to decrease.

So, Fn = m(g-a)

Answer 7

so The scale only reads Fn ?

Answer 8

A scale just tells you how hard it is pushing back on the object that is sitting on it. That is, it tells you the normal force that it is supplying.

Answer 9

I see

Answer 10

so for a) its mg+ma and for b) its mg

Answer 11

mg - ma

A downward acceleration will lower the normal force. Let's look at a free body diagram.



If the normal force is the same as the weight, then the object doesn't accelerate. If, however, the normal force is lowered, then we'd have a net downward force: