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Physics · Grades 7–10

Average speed and average velocity

Choose the correct numerator and include stops in elapsed time.

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Learn the idea

Average speed

Divide total distance by total elapsed time. Include rest periods that occur within the interval.

Average velocity

Divide displacement by elapsed time. Direction changes can make it differ greatly from speed.

Unequal stages

Do not average two speeds directly unless the time weighting justifies doing so.

Units

Metres per second is an SI speed unit. Convert distance and time consistently before dividing.

Worked examples

A walker covers 90 m in 30 s. Find average speed.

90 ÷ 30 = 3 m/s.

3 m/s

A robot goes 12 m out and 12 m back in 8 s. Find average velocity for the full trip.

Its displacement is zero, so 0 ÷ 8 = 0 m/s.

0 m/s

Check your understanding

  1. For that 12 m out and 12 m back trip in 8 s, find average speed.

  2. Which interval belongs in average speed for an entire outing?

    1. Moving time only in every case
    2. The shortest moving stage
    3. All elapsed time, including stops

  3. Why can a round trip have zero average velocity but nonzero average speed?

Answers and marking notes

Answers and marking notes
  1. 3 m/s

    Total distance is 24 m; 24 ÷ 8 = 3 m/s.

  2. All elapsed time, including stops

    Average speed for the whole interval includes the whole elapsed time.

  3. Displacement is zero while total distance is positive.

    Look for the different numerators rather than claiming the object never moved.

Further reading and review references

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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.