You are Here: Home >< Physics

# Calculating uncertainty when subbing in to a formula?

Announcements Posted on
Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
1. The volume V of a cylinder of height h and radius r is given by the expression V = πr2h.
In a particular experiment, r is to be determined from measurements of V and h. The uncertainties in V and in h are :V ±7%, h ± 3%.
Why this is 5% not 10%?
2. I think that the percentage uncertainty should be 10%. Who says that it is 5%? Is this a past paper we can look at?
3. (Original post by Pangol)
I think that the percentage uncertainty should be 10%. Who says that it is 5%? Is this a past paper we can look at?
Maybe so. It's from some online revision notes on the 7th page, it gives the answer below.

http://mrsmithsphysics.weebly.com/up...ties_notes.pdf
4. (Original post by Louiskn)
The volume V of a cylinder of height h and radius r is given by the expression V = πr2h.
In a particular experiment, r is to be determined from measurements of V and h. The uncertainties in V and in h are :V ±7%, h ± 3%.
Why this is 5% not 10%?
Hello there,

If we rearrange the equation, we find that . . .

. . . or alternatively . . .

This implies that the percentage uncertainty, , of the radius can be calculated by . . .

. . . since the percentage uncertainty within the parentheses is multiplied by the exponent. Therefore . . .

. . . has a percentage uncertainty of .

I hope that this has been helpful.
5. (Original post by Smithenator5000)
Hello there,

If we rearrange the equation, we find that . . .

. . . or alternatively . . .

This implies that the percentage uncertainty, , of the radius can be calculated by . . .

. . . since the percentage uncertainty within the parentheses is multiplied by the exponent. Therefore . . .

. . . has a percentage uncertainty of .

I hope that this has been helpful.
Perfect! Thanks!
6. Ah - the lack of formatting in the question made me look right through the square (I thought it was just a 2). Nice explanation!
7. (Original post by Louiskn)
Perfect! Thanks!
You're very welcome.
8. (Original post by Pangol)
Ah - the lack of formatting in the question made me look right through the square (I thought it was just a 2). Nice explanation!
Thank you,

Yes, indeed- if the equation was as it was written in the question post, then you would have been correct.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: June 5, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Who is getting a uni offer this half term?

Find out which unis are hot off the mark here

### Brexit: Why you should study law

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.