levo

Space, Time and Motion

Describing motion: displacement, velocity and acceleration

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on average and instantaneous velocity, average speed against average velocity, (u + v)/2 and why it needs constant acceleration, uniform acceleration and its unit, cyclist and train calculations, negative acceleration, zero velocity with non-zero acceleration, choosing a positive direction, and finding instantaneous velocity from readings.

What you need to know

Answer each one before you reveal it.

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Key term

Define average velocity.

Key term

Define instantaneous velocity.

Key term

How does average speed differ from average velocity for a journey that is not in a straight line?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define average velocity.

    Model answer

    Average velocity is the total displacement divided by the total time taken.

  2. 2

    Define instantaneous velocity.

    Model answer

    Instantaneous velocity is the rate of change of displacement at a particular moment, the value approached as the time interval is made vanishingly small.

  3. 3

    How does average speed differ from average velocity for a journey that is not in a straight line?

    Model answer

    Average speed uses the whole distance travelled, so it is greater than the magnitude of the average velocity, which uses only the displacement.

  4. 4

    Give the average velocity of a body that accelerates uniformly from u to v.

    Model answer

    The average velocity is (u + v)/2.

  5. 5

    Why does the relationship average velocity = (u + v)/2 hold only for constant acceleration?

    Model answer

    Only if the velocity changes at a steady rate is the mean of the initial and final values equal to the true average over time.

  6. 6

    A cyclist speeds up uniformly from 4.0 m s⁻¹ to 10.0 m s⁻¹ in 12 s. Calculate the acceleration.

    Model answer

    The acceleration is (10.0 − 4.0)/12 = 0.50 m s⁻².

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