4.4. Speed http://www.ck12.org
4.4 Speed
- Define speed, and give the SI unit for speed.
- Show how to calculate average speed from distance and time.
- Describe instantaneous speed.
- Show how to calculate distance or time from speed when the other variable is known.
Did you ever play fast-pitch softball? If you did, then you probably have some idea of how fast the pitcher throws
the ball. For a female athlete like the one in the opening image, the ball may reach a speed of 120 km/h (about 75
mi/h). For a male athlete, the ball may travel even faster. A fast-pitch pitcher uses a “windmill” motion to throw the
ball. This is a different technique than other softball pitches, and it explains why the ball travels so fast.
Introducing Speed
How fast or slow something moves is itsspeed. Speed determines how far something travels in a given amount of
time. The SI unit for speed is meters per second (m/s). Speed may be constant, but often it varies from moment to
moment.
Average Speed
Even if speed varies during the course of a trip, it’s easy to calculate the average speed by using this formula:
speed=
distance
timeFor example, assume you go on a car trip with your family. The total distance you travel is 120 miles, and it takes 3
hours to travel that far. The average speed for the trip is: