A question on percentage uncertainty

  • Thread starter Mr Bob555
  • Start date
  • #1
Mr Bob555
3
0

Homework Statement



Say we have the equation D2 = C x N where C = A2.
Using a graphical method to find the % uncertainty of C (from graph of D2 against N) which involves working out the gradient which equals C and so forth. This value is 25%.
The question is "what is the percentage uncertainty of A?"

This is actually a question that appeared in an A-level paper i did today and it was the only one that really bamboozled me :confused:.

Homework Equations




The Attempt at a Solution


I realize you don't divide by two;my reasoning being that when asked to calculate the % uncertainty of r in say the equation V = 4/3 x π x r3 the question doesn't explicitly state r3.

So i think that it is between doubling 25% or just leaving it. Now i think it was the latter but in the exam I did indeed double it to 50%.
 

Answers and Replies

  • #2
Simon Bridge
Science Advisor
Homework Helper
17,874
1,657
You are propagating errors of form ##z=x^a## knowing ##x\pm xp_x/100## where ##p_x## is the known percentage error.
So what is the rule for propagating the error on a power?
 
  • #3
Mr Bob555
3
0
You are propagating errors of form ##z=x^a## knowing ##x\pm xp_x/100## where ##p_x## is the known percentage error.
So what is the rule for propagating the error on a power?
Multiply the percentage uncertainty by whatever the variable is to the order of?
 
  • #4
Mr Bob555
3
0
Anyone else?
 
  • #5
Simon Bridge
Science Advisor
Homework Helper
17,874
1,657
What's wrong with the reply you got so far?
 

Suggested for: A question on percentage uncertainty

  • Last Post
Replies
7
Views
347
Replies
4
Views
714
  • Last Post
Replies
13
Views
408
Replies
7
Views
867
  • Last Post
Replies
3
Views
309
Replies
21
Views
2K
  • Last Post
Replies
4
Views
288
  • Last Post
Replies
30
Views
517
  • Last Post
Replies
3
Views
385
Top