Source: Speed Limit Sign,
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As you learned in section 1, speed is a type of rate. Rate can be defined as a ratio that compares, by division, two quantities having different attributes. Speed compares distance and time by division. Have you ever been asked to define the term speed? If so, you would probably answer, "how fast an object is moving." More specifically, speed is the distance an object travels in a given amount of time.

If the speed of an object does not change during its motion, the object is traveling at a constant speed. Most objects do not move at a constant speed. Most moving objects change speed many times while in motion. The average speed of an object tells you the average rate at which it covers a distance.

Let's pretend you went on a walk through your neighborhood. You walk 5 km to the neighborhood park, and it takes you 45 minutes. You stop and rest for 25 minutes at the park, and then you walk 5 km back home. It takes you 50 minutes to walk back home from the park.

What was your average speed? How is average speed calculated?

To find the average speed, you divide the total distance by the total time. The formula for average speed is

Let's look again at the sample problem from above.

Pretend you went on a walk through your neighborhood. You walk 5 km to the neighborhood park, and it takes you 45 minutes. You stop and rest for 25 minutes at the park, and then you walk 5 km back home. It takes you 50 minutes to walk back home from the park.

First, determine what the total distance. What was the total distance? Interactive popup. Assistance may be required. Check your Answer The problem says you walked 5 km to the park and 5 km back home, so the total distance was 10 km.

Second, determine the total time. What was the total time? Interactive popup. Assistance may be required. Check your Answer It took 45 minutes to get to the park plus a 25 minute rest and the 50 minutes home. That is a total of 120 minutes or 2 hours.

Calculate the average speed. What is the average speed? Interactive popup. Assistance may be required. Check your Answer Average Speed = 10 km over 2 hours 10 km 2 hours
Average Speed = 5km over hour km hour

When you think about the units for speed, you probably think of miles per hour (miles/hr). In science, speed is expressed as kilometer/hour (km/hr) or as meters/second (m/s).

Watch this vide to learn more about average speed and see more sample problems.