Physics question on motion help badly! (1 Viewer)

[jp92]

I need help on this physics question on motion, got to do with an elevator. Is very confusing:

A physics teacher decides, just for fun, to use bathroom scales (calibrated in newtons) in an elevator. The scale provide a measure of the force with which they push up on the teacher. When the lift is stationary the reading on the scales in 823N. What will be the reading on the scales when the elevator is:

(a) Moving upwards at constant speed of 2.0m/s
(b) Accelerating downwards at 2.0m/s
(c) Accelerating upwards at 2.0m/s

Answers
(a) 700N
(b) 560N
(c) 840N

The furthest i can get is finding his mass since
W = MG
823N = Mass times 9.8 (gravity)
M= 84KG (rounded off to nearest kg)

I tried figuring it out, kept getting wrong answers like usual lol
Anyway, any help is much appreciated.

 

[annabackwards]

You should know that when an object moves down it is moving with gravity so acceleration should be positive. But we are interested in what the force is on the scales which is being pushed UP due to the normal reaction force. Therefore when the elevator is moving up, acceleration is positive but when moving down, the acceleration is taken to be negative.

As a result, the teacher is seems to gain weight when the elevator accelerates upwards and loses weight when the elevator gains downwards.


a/
As it is moving at a constant speed, there is no extra acceleration. so the weight would be the same as when it is stationary. That is, 823N.

b/
If the elevator is moving down with acceleration a, then

F = M(g-a)
=84(9.8-2)
=655.2N

c/
Suppose the elevator is moving up with acceleration a.
F-Mg = Ma
F = M(g+a)
=84(9.8+2)
=991.2N

Those are my answers.

Are you sure the answers your looking at aren't for another question? The teacher would have to weigh about 70kg for the answers you've given to be right.

 

[jp92]

hey annabackwards,
yeh exactly thats what i was thinking except the back of the book doesnt like it
yeh im pretty sure ive looked at the right answers, ive looked close to 100 times lol
maybe the book is wrong
anyway, ill just have to ask my teacher when i go back to school.
thanks for the help better get back to my boring assignment on search for identity