Source: Limit Sign, USA Today

You are probably familiar with the concept of speed. Speed is a type of rate. A rate tells you the amount of something that occurs during a given unit of time. If you know the distance an object travels in a certain amount of time, you know the speed of the object. For example, if a car travels 70 miles in one hour, the car is traveling at a speed of 70 miles/hour (miles per hour).

If the speed of an object does not change during its motion, the object is traveling at a constant speed. Most cars are equipped with cruise control. Cruise control allows a car to travel at a constant speed.

Source: Marathon Runners, Discovery News

Most objects do not move at a constant speed very long. Most moving objects change speed many times while in motion. The marathon runners pictured here do not run at a constant speed during the entire race. At the starting line, there are many runners crowded together, so the runners are forced to move slowly. As the crowd thins out, a runner can increase his or her speed. The runner may slow down as he or she climbs a hill and increases speed going down the hill. When the race is over, a runner may calculate his or her average speed. The average speed of an object tells you the average rate at which it covers distance. You will learn more about average speed and how to calculate it in lesson 4 of this module.