relative vs absolute uncertainty

[bale]

I did a practicle and claculated time, then i had to do some more calculations to calculate 1/time so i had to calculate the precentage uncertainty. My question 'is the uncertainty absolute or relative and why...how do i know which one it is?'

[theboro76]

As a general rule % uncertaintity is relative, as it is in relation to the measurement.

A uncertainty that states a specific value, then it is absolute as it doesn't change depending on the specific value.

i.e. Relative uncertaintity of 20%. for a value of 100, the uncertaintity is +/- 20 units (20% of 100). for a result of 200 the uncertaintity the uncertaintity is +/- 40 units(20% of 200).

For absolute uncertaintity of +/-20 units. In a value of 100 the uncertaintity is 20 units. for 200 units the uncertaintity is still +/- 20 units.

Rember by relative uncertainity is relative to the value, so it increses and decreases, as it is a %. Absolute is absolute so it never changes, as it is a certain number of units.

[bale]

As a general rule % uncertaintity is relative, as it is in relation to the measurement.

A uncertainty that states a specific value, then it is absolute as it doesn't change depending on the specific value.

i.e. Relative uncertaintity of 20%. for a value of 100, the uncertaintity is +/- 20 units (20% of 100). for a result of 200 the uncertaintity the uncertaintity is +/- 40 units(20% of 200).

For absolute uncertaintity of +/-20 units. In a value of 100 the uncertaintity is 20 units. for 200 units the uncertaintity is still +/- 20 units.

Rember by relative uncertainity is relative to the value, so it increses and decreases, as it is a %. Absolute is absolute so it never changes, as it is a certain numbe

but i have seen something in ibid book wich says that relative uncertainty *100=PERCENTAGE UNCERTAINTY

could you tell me the difference between them

[theboro76]

I guess what you have to understand is the sylubuss actually seperates percent and fractional uncertaintity

1.2.10 State uncertainties as absolute, fractional and percentage uncertainties.

So relative uncertaintity, as described in your textbook is fractional uncertaintity, as described in different forms. So it will +/- 0-point-somthing (hopefully, unless you are extremely inaccurate in an experiment), rather than a %.

So, it is my understanding that they are BOTH relative uncertaintities, just different forms of it. For example, Catholcism and protestantism are BOTH forms of christianity, just different forms.

The important point out of this is that for adding and subtracting you add absolute values. For multiplying and dividing you add relative uncertaintities, for this I assume that you should be able to add fractional and percentage for this purpose. However I have attached a link that should make it more clear for you, you have to scroll down a bit. They recommend adding % uncertainties.

http://www.ibguides.com/physics/notes/measurement-and-uncertainties

[susanne]

in short, when an uncertainty is expressed as a percentage of a measurement, it is relative,

when it's in the same unit as the measurement, it's absolute

you must always express uncertainty in absolute when giving a finalized answer