Hi, I am Peter Edwards from Bluetutors and we teach children of all ages right from primary school to degree level and we find the highest quality teacher. Today I am going to teach you some maths. So we are now going to go though how to calculate uncertainty. Uncertainty depends on what you have measured, whatever you are measuring with, so in this case we are going to use a piece of wood and a ruler. Obviously it is magnified - that's not really one centimetre. So when you are looking at this with a ruler you would generally measure to the nearest millimetre, and that's where uncertainty comes from. So in this case if we look at this that is either eight point three (8.3) or eight point four(8.4) centimetres. So let's look at where the uncertainty comes from. Let's say it was eight point three (8.3) centimetres. We want to know the minimum or maximum that piece of wood could be given that measurement. So what we say is it could be as low as eight point two three (8.23) centimetres or as big as eight point three five (8.35) centimetres. If it were any lower than eight point two five (8.25) centimetres then we would say eight point two (8.2) centimetres, and if was any bigger than eight point three five (8.35) then we would say it was 8.4 cm. So in this case we would say we have measured this piece of wood to be eight point three (8.3) centimetres plus or minus point zero point five centimetres, and that's how you calculate uncertainty. Now what we could do is we could add together two pieces of wood and decide about the uncertainty of that edition. So let's say we measured something to eight point three centimetres plus or minus point zero five centimetres, and something else to let's say fifteen point four meters. Now in this case the uncertainty is going to be different because it's measured to that one decimal place of a meter, so that's going to be plus or minus zero point zero five meters. So if we were to add those two measurements together, what we would end up with is fifteen point four meters plus eight point three centimetres, and we have to add the uncertainties. And that is how you work out the uncertainty of a number and also when you add them together. .

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