Quantities in physics generally have units, for example, ms–1, and a common error is to forget to put the correct unit after completing a calculation. However, there are a number of quantities used in AS and A2 physics that are ratios and do not have units. Examples of these are tensile strain, which is the ratio of two lengths, or refractive index, which is the ratio of two speeds. In the case of tensile strain, not only is it a ratio of lengths but also is commonly expressed as a percentage, which can be a trap for the unwary.
As in the last Mathskit, PHYSICS REVIEW Vol. 19, No. 1, some of the examples will be illustrated with a commonly used student calculator (Figure 1). Other calculators are available, and most will have the same or similar functions.
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