Speed vs Velocity
8 Minutes
Speed and velocity for students: learn how speed and velocity differ, how to calculate them, and what average speed means. A physics lesson in English.
- How to calculate speed using distance and time in a 100-meter race
- Finding velocity and speed for a trip between home and school
- Calculating average speed and understanding instantaneous speed
What this lesson teaches
Speed is the rate that measures how fast an object moves along a path, while velocity measures how fast an object is displaced in a specific direction.
Differences between speed and velocity
- Speed depends on the total distance traveled divided by the time taken to cover that distance.
- Velocity depends on displacement, which is the shortest direct path between the starting point and finishing point, divided by time.
- Speed is a scalar quantity because it only has magnitude and does not depend on direction.
- Velocity is a vector quantity because it requires both a magnitude and a clear direction of motion.
Calculating speed and velocity
In a simple 100-meter sprint finished in 15 seconds, dividing 100 meters by 15 seconds gives a speed of 6.67 meters per second. On a trip to school, the actual curved road may cover a distance of 500 meters, while the straight-line displacement is only 200 meters. If that trip takes 20 minutes, multiplying 20 by 60 gives 1,200 seconds. Dividing 500 meters by 1,200 seconds gives a speed of 0.416 meters per second, while dividing the 200-meter displacement by 1,200 seconds gives a velocity of 0.167 meters per second.
Average speed and zero velocity
When motion is broken into separate stages, average speed is calculated by dividing the total distance traveled by the total time taken. For example, traveling 500 meters to school in 20 minutes, staying 5 minutes, and traveling 500 meters back home in 10 minutes gives a total distance of 1,000 meters. The total time taken is 35 minutes, which multiplies by 60 to equal 2,100 seconds. Dividing 1,000 meters by 2,100 seconds gives an average speed of 0.476 meters per second. Because the trip finishes at the starting point, the total displacement is zero meters, which makes the velocity for the entire round trip zero.
Instantaneous speed and velocity
Instantaneous speed is the speed of an object measured at one specific frozen moment in time, such as the value shown on a bike speedometer. A speedometer measures motion by taking two points that are as close together as possible and dividing that small distance by the time taken. When looking extremely closely between two nearby points, any curved path forms a straight line. Because the distance and the displacement between these very close points are identical, instantaneous speed and instantaneous velocity are equal.