Velocity and speed

​​What is the difference between velocity and speed? Velocity is the vector equivalent of speed. Since speed is the distance travelled per unit time, this means that velocity is displacement travelled per unit time.

If you're estimating how long a journey will take, you will probably use an average velocity. However, sometimes instantaneous velocity is more important. 


Key Concepts

Velocity is defined as the rate of change of displacement:

\(v={\Delta s \over \Delta t}\)

The derived SI unit of velocity is m s-1.

Speed is defined as the rate of change of distance:

\(\text {speed} = {\text {distance} \over \text {time}}\)


Speed is a scalar and velocity is a vector. This means if you tavel in a complete circle at constant speed your velocity is constantly changing and your average velocity will be zero.

Watch this clip and notice the changing direction of the velocity arrow.

Essentials

Instantaneous velocity

Any moving object has an instantaneous velocity.

Instantaneous velocity is the velocity you would calculate if you divided motion into small steps and calculated the step length divided by time taken: \(v_{\textrm {instantaneous}}={\delta s \over \delta t}\).

The direction of the velocity is the same as the direction of the change in displacement.

Constant velocity

Velocity has direction so is represented by a vector. The length of the vector is proportional to the magintude of the velocity. In 1-dimension the sign gives the direction (positive is right).

It is conventional to take the initial velocity of a body as the positive direction, but this is not essential, provided you are consistent within a question.

Average velocity

 If velocity is constant then average velocity = instantaneous velocity. If velocity is not constant they will be different, average velocity is simply the total displacement / total time. On a graph, these would be found from the final and intial points, ignoring whatever happens between.

Components of velocity

As with all vectors, velocity can be split into two perpendicular components, usually horizontal and vertical.

When solving problems in 2D we can treat the components separately.

Test Yourself

Use quizzes to practise application of theory. 


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