READ: Speed and Velocity

READ: Speed and Velocity

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.

Speed

How fast or slow something moves is its speed. 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. Even if speed varies during the course of a trip, it’s easy to calculate the average speed by using this formula:

speed=distancetime

For 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:

speed=120 mi3 h=40 mi/h

Velocity

Ramey and her mom were driving down this highway at 45 miles per hour, which is the speed limit on this road. As they approached this sign, Ramey’s mom put on the brakes and started to slow down so she could safely maneuver the upcoming curves in the road. This speed limit sign actually represents two components of motion: speed and direction.

Speed and Direction

Speed tells you only how fast or slow an object is moving. It doesn’t tell you the direction the object is moving. The measure of both speed and direction is called velocity. Velocity is a vector. A vector is measurement that includes both size and direction. Vectors are often represented by arrows. When using an arrow to represent velocity, the length of the arrow stands for speed, and the way the arrow points indicates the direction. 

Calculating Average Velocity

You can calculate the average velocity of a moving object that is not changing direction by dividing the distance the object travels by the time it takes to travel that distance. You would use this formula:

velocity=distancetime

This is the same formula that is used for calculating average speed. It represents velocity only if the answer also includes the direction that the object is traveling.

Let’s work through a sample problem. Toni’s dog is racing down the sidewalk toward the east. The dog travels 36 meters in 18 seconds before it stops running. The velocity of the dog is:

velocity=distancetime=36 m18 s=2 m/s east

Note that the answer is given in the SI unit for velocity, which is m/s, and it includes the direction that the dog is traveling.

Q: What would the dog’s velocity be if it ran the same distance in the opposite direction but covered the distance in 24 seconds?

A: In this case, the velocity would be:

velocity=distancetime=36 m24 s=1.5 m/s west

Summary

• How fast or slow something moves is its speed. The SI unit for speed is meters per second (m/s).
• Average speed is calculated with this formula: speed=distancetime
• Velocity is a measure of both speed and direction of motion. Velocity is a vector, which is a measurement that includes both size and direction.

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