You are Here: Home

Physics and electronics discussion, revision, exam and homework help.

Announcements Posted on
Find out how cards are replacing warnings on TSR...read more 03-12-2013
Say if a micrometer's precision is 0.01mm then is it's uncertainty in measuring also 0.01mm or is it 0.005mm, half of the precision?

Also say if the % uncertainty of X is 5% then what is the %uncertainty of 7X?
Is it simply 7*5 which is 35%?

Thanks I've been looking for answers to these two problems but can't find them. Got a test on Tuesday!
2. Re: Urgent uncertainties help please!
Bump!
3. Re: Urgent uncertainties help please!
(Original post by Nodes Of Ranvier)
Say if a micrometer's precision is 0.01mm then is it's uncertainty in measuring also 0.01mm or is it 0.005mm, half of the precision?

Also say if the % uncertainty of X is 5% then what is the %uncertainty of 7X?
Is it simply 7*5 which is 35%?

Thanks I've been looking for answers to these two problems but can't find them. Got a test on Tuesday!
There is no hard and fast rule.
Most commonly, if a scale has a minimum division, like a rule has 1mm, you give the precision as ±half that value. ie. ±0.5mm
But it depends on how you use the instrument and what you are measuring. You may decide you cannot read to that precision and give it as ± 1mm.
That would be the largest value you would normally expect to see for the precision of that instrument.
The uncertainty in measuring something is not the same as the precision. Precision is to do with the scale, but uncertainty is about how you make the measurement.
The classic example is timing with a stopwatch. The precision of the watch could be ±0.01s but if you were timing something you would need to include your reaction time pressing the button. This could be as much as ± 0.2s
This would be the uncertainty of the measurement.

If the % error in X is 5% the % error in 7X is also 5%
The actual (absolute) error will of course be 7 times greater.
4. Re: Urgent uncertainties help please!
(Original post by Stonebridge)
There is no hard and fast rule.
Most commonly, if a scale has a minimum division, like a rule has 1mm, you give the precision as ±half that value. ie. ±0.5mm
But it depends on how you use the instrument and what you are measuring. You may decide you cannot read to that precision and give it as ± 1mm.
That would be the largest value you would normally expect to see for the precision of that instrument.
The uncertainty in measuring something is not the same as the precision. Precision is to do with the scale, but uncertainty is about how you make the measurement.
The classic example is timing with a stopwatch. The precision of the watch could be ±0.01s but if you were timing something you would need to include your reaction time pressing the button. This could be as much as ± 0.2s
This would be the uncertainty of the measurement.

If the % error in X is 5% the % error in 7X is also 5%
The actual (absolute) error will of course be 7 times greater.
Cheers mate

## Step 2: Register

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

this is what you'll be called on TSR

2. this can't be left blank

never shared and never spammed

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. By completing the slider below you agree to The Student Room's terms & conditions and site rules

2. Slide the button to the right to create your account

You don't slide that way? No problem.

Last updated: April 15, 2012
Study resources