C
CraftExam
medium difficulty7 min

Speed and Velocity

Speed is the distance travelled per unit time. Velocity is the displacement per unit time in a specified direction.

In one line: Speed uses the path; velocity uses the arrow.

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Understand it

Average speed depends on total distance, while average velocity depends on total displacement. Both have SI unit m/s, but velocity can be positive, negative, or zero depending on direction and displacement.

Example

If a runner completes one round of a circular track and returns to the start, average speed is not zero, but average velocity is zero.

Rule to remember

Average speed = total distance / total time.

Average velocity = displacement / total time.

SI unit for both is metre per second, written as m/s.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A cyclist travels 100 m east in 20 s and then 100 m west in 20 s.

  1. 1Total distance = 200 m, time = 40 s, average speed = 5 m/s.
  2. 2Displacement = 0, so average velocity = 0 m/s.

Method you can reuse

  1. 1For average speed, add all distances and divide by total time.
  2. 2For average velocity, find displacement from start to finish and divide by total time with direction.

Visual tip

A circular track or back-and-forth line diagram can show non-zero distance with zero displacement.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why can average velocity be zero even when average speed is not zero?

Think first, then show answer
Average velocity can be zero when final position is the same as initial position, because displacement is zero, while distance travelled may still be positive.

Common wrong answer

Writing average velocity as 5 m/s in the worked example.

Why it loses marks: That uses distance instead of displacement.

Don't mix this up

Students often use total distance while calculating average velocity, especially in round-trip questions.

Exam tip

Read whether the question asks speed or velocity; then choose distance for speed and displacement for velocity.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions often give a round trip, a return journey, or a direction change and ask for average speed and average velocity.
  1. 1

    Enough for 1 mark

    Speed is the distance travelled per unit time. Velocity is the displacement per unit time in a specified direction.

  2. 2

    Add for 2 marks

    Example: If a runner completes one round of a circular track and returns to the start, average speed is not zero, but average velocity is zero.

  3. 3

    Add for 3 marks

    Average speed depends on total distance, while average velocity depends on total displacement.

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