Percentage Uncertainty

  • #1
Two lengths, a and b, are measured to be 51+/-1cm and 49+/-1cm respectively. In which of the following is the percentage uncertainty the largest?

A a+b
B a-b
C a x b
D a/b

The answer should be b but I can't figure out why. I know that for / and x you add the percentage errors so if the error is approx 2% then each of C and D have errors of 4% of their answers ie

C 51 x 49 = 2499 and 4% is ~100
D 51/49 = 1 and 4 % is 0.04

I think that for + and - you add absolute uncertainties ie 0,02 + 0.02 = 0.04 ie

A 51 + 49 = 100 and 100 x 0.04 = 4
B 51 - 49 = 2 and 2 x 0.04 = 0.08

But that makes C the largest error.

What am I missing here? It's supposed to be multiple choice, non calculator so I'm probably overthinking it?
 

Answers and Replies

  • #2
BvU
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Don't see how absolute uncertainties of 1 each can add up to 0.04 ...
 
  • #3
haruspex
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C 51 x 49 = 2499 and 4% is ~100
100 is the absolute error. You are only interested in percentage errors, so stop the calculation at 4%.
BvU said:
Don't see how absolute uncertainties of 1 each can add up to 0.04 ...
I'm guessing calculator20 is considering +/-1 as a range of 2.
 
  • #4
BvU
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I think that for + and - you add absolute uncertainties ie 0,02 + 0.02 = 0.04
@calc: 0.02 is the relative error! You want the absolute error here !

for error read uncertainty - saves confusion
 
Last edited:
  • #5
HallsofIvy
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Two lengths, a and b, are measured to be 51+/-1cm and 49+/-1cm respectively. In which of the following is the percentage uncertainty the largest?

A a+b
Then a+ b= 100 +/- 2 cm (NOT 4) so the percentage error is 2/100= 2%

B a-b
a- b= 2+/- 2 cm so the percentage error is 2/2= 100%

C a x b

D a/b
The percentage errors for a and b separately are 1/51= 0.0196 or about 2% and 1/49= 0.02041 or about 2% so the percentage errors of a x b and a/b are 2+ 2= 4%

The answer should be b but I can't figure out why. I know that for / and x you add the percentage errors so if the error is approx 2% then each of C and D have errors of 4% of their answers ie

C 51 x 49 = 2499 and 4% is ~100
D 51/49 = 1 and 4 % is 0.04

I think that for + and - you add absolute uncertainties ie 0,02 + 0.02 = 0.04 ie

A 51 + 49 = 100 and 100 x 0.04 = 4
B 51 - 49 = 2 and 2 x 0.04 = 0.08

But that makes C the largest error.

What am I missing here? It's supposed to be multiple choice, non calculator so I'm probably overthinking it?
 
  • #6
BvU
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@Ivy: I thought we wanted calc to learn something, not have the answers dumped in his lap !
 
  • #7
BvU
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All in good spirit, of course...
 
Last edited:
  • #8
Thank you everyone. I'd gone with fractions rather than whole values. I can see where things went wrong now. Many thanks!
 

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