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How to Use the Speed Formula

By Kathy Foust

Use this Speed Formula Study to increase understanding of the speed formula as it applies to motion.

What is Speed?

Speed is one way to measure one type of motion as described in the motion study guide that discusses speed, acceleration, velocity and motion in general.

Speed is a scalar quantity, which means it does not measure direction, only rate of movement.

Instantaneous speed is a rate measured by a device such as a speedometer. The speedometer measures the rate at which the vehicle is moving at that moment. To measure the average rate of speed however, one must use the formula below. Average speed is the distance traveled divided by the time it takes to travel that distance. See below for a more detailed look at this formula.

Speed Formula

The speed formula is the formula used to measure the average speed that an object is moving. The formula is expressed below.

Average speed = distance traveled/ time to travel distance. Average speed is also given if the rate of speed remains constant during the entire period of movement. This is a given, but can also be checked by using the average speed formula.

Let's walk through an example of this formula. Assume that you left your house and took an hour to drive to your Grandmother's house, then took another 45 minutes to drive to your uncle's house. The entire drive took 1 hour and 45 minutes and covered a distance of 120 miles. In any formula, the units must remain constant so we need to change the 1 hour and 45 minutes to one type of unit. For this example we will change the amounts to hours. since 45 minutes is 3/4 or 75% of an hour, the result is that it took 1.75 hours to travel the entire distance.

average speed = 120 mi/1.75 h (Where mi=miles and h= hours)

Now do the math. 120 divided by 1.75 equals 68.57. If this number were to be rounded off to the nearest mile per hour, it would be written as 69 mph.

The formula is the same no matter what the units are. Speed units are generally written in miles per hour or meters per second. So, when using this formula, be sure to first change the amounts in the formula into standard units.