What is the UNCERTAINTY of this value?

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nonipaify
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A figure shows two analogue scales of mass/g with the smallest division of 1 gram. a) shows the mass of a chemical before being tipped into a beaker of water and b) shows the mass after the chemical has been tipped. The reading on a) is 38 and on b) is 23. It then asks to calculate the mass of the chemical used and the uncertainty in this value.

The mass is calculated to 15 g. NOW, uncertainty in a single reading from an analogue scale is 0.5*smallest division and so is +- 0.5 g. Since we are taking two readings, the uncertainty should add upto +- 1 g but the answer at the back states it as 15 +-2 g? Is it because we are actually taking four readings (including the 0 in both diagrams)?
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Stonebridge
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(Original post by nonipaify)
A figure shows two analogue scales of mass/g with the smallest division of 1 gram. a) shows the mass of a chemical before being tipped into a beaker of water and b) shows the mass after the chemical has been tipped. The reading on a) is 38 and on b) is 23. It then asks to calculate the mass of the chemical used and the uncertainty in this value.

The mass is calculated to 15 g. NOW, uncertainty in a single reading from an analogue scale is 0.5*smallest division and so is +- 0.5 g. Since we are taking two readings, the uncertainty should add upto +- 1 g but the answer at the back states it as 15 +-2 g? Is it because we are actually taking four readings (including the 0 in both diagrams)?
Uncertainty can be +- half the scale division but can also be +- the whole scale division, in this case +- 1g

There is no hard and fast rule, it depends on the instrument and how it is read.
The question is presumable taking the latter as the uncertainty, as they add in this case.
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