Newton's Second Law

nguyentech

This question has been troubling me... :

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

a) moving upwards at a constant speed of 2.0 m/s/s
b) accelerating downwards at 2.0 m/s/s
c) accelerating upwards at 2.0 m/s/s

The apparent answers at the back of the textbook read:
a) 700 N
b) 5.6 x 10^2 N
c) 8.4 x 10^2 N

I think they are wrong?

a) 823 N
b) 655 N
c) 991 N

Are my answers correct?

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dav2008

Gold Member
How did you arrive at your answers? That way we can see if you are faulted in your method or not.

Sting

Yes, place where your work here so we could determine where any potential problem went.

nguyentech

For lift accelerating upwards : T = mg + ma, therefore answer = 84*9.8 + 84*2 = 991
For lift accelerating downwards : T = mg - ma, therefore answer =
84*9.8 - 84*2 = 655

dav2008

Gold Member
Ok, well for 1, i assume it said moving at a constant 2 m/s not m/s/s...in that case, you are right, the Normal force(what the scale reads) is the same

Ok, your equation is Fn-Fg=m*a

For 2 m/s^2 upward, you have Fn-823=(823/9.8)*2

Which means Fn=991...which is what u got....

And for downward Fn-823=(823/9.8)*-2 which gives Fn= 655

Either were both making the same mistake (I havent done these in a while lol) or the book is just wrong..

Lemme try to use g=10 for the calculations..
Fn=987 and Fn=658..yea still close to our answers and far from the books..

I think the fact that they say that with constant speed, the normal force changes, says that the book is broken

enigma

Staff Emeritus
Gold Member
Yeah, the book is wrong.

Eyesee

the book is wrong

dav2008

Gold Member
This is why i never trust books.

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