A passenger jet accelerates from rest along a runway and reaches a take-off speed of 80 m/s in 40 seconds. After take-off, the jet climbs at a constant speed but changes direction as it follows a curved flight path. Explain why the jet has a changing velocity during the climb even though its speed remains constant. Then describe what the term 'acceleration' tells us about the jet during its take-off run along the runway.

Eduqas GCSE Physics — 4.1 Speed, velocity and acceleration · Explain and Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A jet’s velocity is a vector, so it has both magnitude (speed) and direction. During the climb the jet’s speed is constant but its direction changes as it follows a curved path, so its velocity vector changes – therefore the jet is accelerating.

Acceleration is the rate of change of velocity. During the take‑off run the jet’s velocity increases from 0 to 80 m s⁻¹, so the jet has a positive acceleration.

Examiner tips

  • Use the term ‘vector’ when describing velocity; mention both magnitude and direction.
  • Explain that a change in direction alone changes velocity even if speed is constant.
  • State that acceleration is the rate of change of velocity and that the jet’s velocity is increasing during take‑off.
  • Use the word ‘positive’ to describe the direction of the acceleration vector.

Common mistakes

  • Confusing speed with velocity; writing that speed changes during the climb.
  • Forgetting to mention that acceleration is a vector and that the jet’s acceleration is positive during take‑off.
  • Leaving out the fact that the jet’s velocity changes because of the change in direction on the curved path.

Mark scheme (4 marks)

  1. Velocity is a vector quantity / velocity has both magnitude (speed) and direction
  2. Because the direction of travel changes (on the curved path), the velocity changes even though the speed stays the same
  3. Acceleration is the rate of change of velocity
  4. During the take-off run the jet's velocity is increasing (from 0 to 80 m/s) so it is accelerating / the jet has a positive acceleration

Key terms in this question

velocity · speed · acceleration

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