What is the difference between average speed and average velocity?

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Speed is speed, or how fast something is going, while velocity is speed or how fast something is going in a specific direction. Average speed is the speeds added up and divided by number of speeds, and you do the same tning to calculate average velocity.
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Speed is the rate of change in position in a given amount of time.  Thus, speed is measured in units of meters/second (m/s), kilometers per hour (km/hr) and more familiarly miles/hour (mph).  Average speed is then the total distance traveled divided by the total time spent traveling.

Velocity has identical units to speed but the direction of motion must also be specified.  In addition, velocity looks at the displacement (the shortest distance from the start of the motion to the end of the motion).  This can lead to some interesting differences between average speed and average velocity.

Consider, for example, a runner on a 400 meter track.  Suppose the runner completes exactly 4 laps in 6 minutes.  Their average speed is then 1600 m/360 s or 4.44 m/s.  However, there average velocity is zero!  This is because their displacement is zero.

Another example would be if someone drives 300 m east, then 400 m north in a total time of 5 minutes.  Their average speed is 700 m/300 s or 2.33 m/s.  However, their displacement is 500 m from where they started.  Therefore, their average velocity is 500 m/300 s or 1.67 m/s @ 53.13 degrees north of east.

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The basic difference is that speed is a scalar quantity, and velocity is a vector quantity; basically, direction matters when talking about velocity, and it doesn't when we are talking about speed. Speed is how fast something is moving--the rate at which a distance is covered. Velocity measures how fast something changes position. Think of it this way: if you run 100 meters in 10 seconds then immediately run back to your starting point in 10 seconds, your average speed is 10 meters per second--you have covered 200 meters in 20 seconds. Your average velocity is zero--you haven't changed your position after 20 seconds.

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