Differentiate significant figures and non-significant figures?

Answer 1

Significant figures are those that are actually required to specify a quantity, and can be used in working with that quantity. Thus if you are using a ruler with millimeter divisions to measure something, you can only measure to the nearest millimeter. If you were measuring the diameter of a circle in order to calculate the perimeter of the circle, you could look up the value of Pi to as many decimal places as you like, say 10 places, and so your value of the perimeter ( Pi x d) would appear to be extremely accurate having a value quoted to 10 decimal places. But in fact it would only be accurate to the nearest millimeter, as was the diameter you measured, so the decimal places beyond the number of whole millimeters should be discarded, as non-significant. They could be real, but you can't say if they are or not, so best not to quote them.

This is an example of the idea-don't present a result as more accurate than the components which went into it.