^{76}Surely you’ve heard the expression, “Apples and Oranges don’t add up.” Well, pounds per square inch and pounds per
square foot don’t add up either! A general mathematical rule in physics is that any quantities added to or
subtracted from each other must bear the exact same units. This rule does not hold for multiplication or division,
which is why we see units canceling in those operations. With addition and subtraction, no unit cancellation
occurs.