Physics Lesson 3.5.1 - What is Speed? How do we measure it?

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Welcome to our Physics lesson on What is Speed? How do we measure it?, this is the first lesson of our suite of physics lessons covering the topic of Speed and Velocity in 1 Dimension, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

What is Speed? How do we measure it?

Let's try to give an answer to the first question posed in the "Introduction" part through an example.

Eight athletes are participating in a 400 m running race. All of them manage to complete the race. Do you think they are awarded equally by the end of the race? Why?

Obviously, the answer is NO. Each race has its own winners. Sometimes it is enough to complete a task to be the winner (in marathon or rally races) but this is not our case. Since all participants (athletes) are able to complete their race, the jury must use another criterion to make the distinction between the participants and not simply to evaluate the race completion. This criterion consists on completing the race in the shortest time possible. Therefore, a new quantity is introduced here to determine the winner in addition to the running distance. This quantity is the "Time of Motion" (in short Time) which is denoted by "t" and is measured in seconds [s].

When combining the moving distance and time we obtain a new Kinematic quantity known as Speed (v).

By definition, Speed (v) is the Distance travelled by an object in a given time (sometimes we say "in the unit of time" instead of "in a given time").

Mathematically, we can write:

v = s/t

Since (in the SI system of units) the Displacement is measured in metres and Time in seconds, the unit of speed is

Unit of speed = [metre/second] = [m/s]

For example, if the winner completes the 400 m race in 80 seconds, his speed is

v = s/t
= 400m/80s
= 5 m/s

Since Speed is obtained by dividing two scalar quantities such as Distance and Time, it is a scalar quantity as well. Therefore, no direction is involved when discussing about moving speed of an object.

You have reach the end of Physics lesson 3.5.1 What is Speed? How do we measure it?. There are 4 lessons in this physics tutorial covering Speed and Velocity in 1 Dimension, you can access all the lessons from this tutorial below.

More Speed and Velocity in 1 Dimension Lessons and Learning Resources

Kinematics Learning Material
Tutorial IDPhysics Tutorial TitleTutorialVideo
3.5Speed and Velocity in 1 Dimension
Lesson IDPhysics Lesson TitleLessonVideo
3.5.1What is Speed? How do we measure it?
3.5.2The meaning of Average and Instantaneous Speed
3.5.3What is Velocity? How do we measure it?
3.5.4Average and Instantaneous Velocity

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  6. Continuing learning kinematics - read our next physics tutorial: Speed and Velocity in 2 and 3 Dimensions

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