We can use signed numbers to represent the position of an object along a line. We pick a point to be the reference point and call it zero. Positions to the right of zero are positive. Positions to the left of zero are negative.

A number line with the numbers negative 10 through 10 indicated. A point is indicated at zero and is labeled "reference point." Another point is indicated at negative 4 and is labeled "4 units to the left of zero." A third point is indicated at 7 and is labeled "7 units to the right of zero."

When we combine speed with direction indicated by the sign of the number, it is called velocity. For example, if you are moving 5 meters per second to the right, then your velocity is +5 meters per second. If you are moving 5 meters per second to the left, then your velocity is -5 meters per second.

If you start at zero and move 5 meters per second for 10 seconds, you will be 50 meters to the right of zero. In other words,

$\displaystyle 5\boldcdot 10 = 50$

If you start at zero and move -5 meters per second for 10 seconds, you will be 50 meters to the left of zero. In other words,

$\displaystyle \text-5\boldcdot 10 = \text-50$

In general, a negative number times a positive number is a negative number.