Video Transcript
Which of the following is an
example of a quantity as defined in physics? (a) Five kilograms, (b) five, (c)
mass, (d) kilograms.
In everyday life, the word
“quantity” usually means an amount of something. But in physics, quantity has a
slightly different and very specific meaning. A good working definition of the
word “quantity” in physics is something of which there can be an amount. Another way to think about this is
that a quantity is something that we can set out to measure. And the result of that measurement
will be the value of that quantity. Now, the result of a measurement
that is the value of our quantity will always be a number followed by a unit.
This helps us clearly state the
difference between a quantity and the value of a quantity as determined by a
measurement. A quantity is something like time
that we can measure. And the value of that quantity as
determined by a measurement would have units appropriate for time like, say, seconds
or milliseconds and the number of seconds or milliseconds that we measured. Looking at our answer choices, we
see that answer (a) has the form of the result of a measurement, a number times a
unit. So, (a) is not the correct answer
because it is the value of a quantity but not a quantity itself.
Similarly, five is just a
number. And the unit kilograms are
appropriate units for a measurement of the quantity mass. But kilograms are not themselves
that quantity. This leaves answer choice (c),
mass, and indeed mass is a quantity. We might set up an experiment to
measure the mass of an object. And we would get back a specific
value like, say, five kilograms as we saw in answer (a).
