Newton's Second Law and a bathroom scale

  • Thread starter nguyentech
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  • #1
nguyentech
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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?

My answers are:
a) 823 N
b) 655 N
c) 991 N

Are my answers correct?
 

Answers and Replies

  • #2
dav2008
Gold Member
612
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How did you arrive at your answers? That way we can see if you are faulted in your method or not.
 
  • #3
Sting
157
2
Yes, place where your work here so we could determine where any potential problem went.
 
  • #4
nguyentech
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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
 
  • #5
dav2008
Gold Member
612
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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 haven't 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
 
  • #6
enigma
Staff Emeritus
Science Advisor
Gold Member
1,757
17
Yeah, the book is wrong.
 
  • #7
the book is wrong
 
  • #8
dav2008
Gold Member
612
1
This is why i never trust books.
 

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