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October 21, 2014

October 21, 2014

Posted by **Physics** on Friday, July 10, 2009 at 12:29pm.

This is a logical puzzle. Having 75% of the typical weight means experiencing 75% of the typical gravity. The floor must be falling away with an acceleration of 25% of typical gravity.

Here's the question I asked that's from my textbook

A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads only .75 of the person''s regular weight. Calculate the acceleration of the elevator, and find the direction of acceleration.

ok isn't weight by defintion the force of gravity which is equal to mass time gravity how could this every change? I understand that in this situation the net force change but no the actual weight so I'm lost... I'm know the normal force exerted by the scale onto the person would change becasue the acceleration changes and for a brief moment weighs less than she would without going in the elevator...

so I'm the normal force exerted by the scale onto the person would obviously change but how would her weight change?

Can you explain to me how to do this problem or as to why I can do this problem this way because I do not understand it. Thanks

- Physics -
**Count Iblis**, Friday, July 10, 2009 at 1:05pmHer weight doesn't change, but her so-called "apparent weight" does change:

http://en.wikipedia.org/wiki/Apparent_weight

- Physics -
**Marth**, Friday, July 10, 2009 at 1:12pm"how would her weight change?"

The force of weight we sense is actually the normal force, or the force pushing us up. If the elevator accelerates downward, that force is reduced, as is our perception of our weight.

By Newton's 3rd law, the sum of the forces acting on the person equals the mass times acceleration.

sum(F) = ma.

the sum of forces also equals the normal force + the force of weight.

sum(F) = Fn - Fw

When the person is at rest, the sum of the forces = 0.

0 = Fn - Fw

However, when the elevator accelerates downwards, the Fn is only .75 of what it was.

sum(F) = .75Fn + FW

ma = .75mg - mg

a = .75g - g

a = -.25g

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