# A question on percentage uncertainty

Mr Bob555

## 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 .

## 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

Homework Helper
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?

Mr Bob555
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?

Mr Bob555
Anyone else?