Explain why an object moving with constant speed along a curved path must be accelerating.

IB DP Physics Standard Level (2023 syllabus) — A.1 Kinematics · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Velocity is a vector, having both magnitude and direction.
2. On a curved path the direction of velocity changes continuously.
3. A change in velocity (in magnitude or direction) is acceleration.
4. Thus, constant speed but changing direction gives non‑zero acceleration.

Examiner tips

Use the definition of velocity as a vector; mention direction change. Show that acceleration is defined as change in velocity, not just speed. Keep answer concise and directly linked to the marks.
Use numbered points to match the four marks and show clear logical flow.

Common mistakes

Confusing speed with velocity and ignoring direction. Claiming that constant speed means no acceleration. Using vague language like "the object is speeding up" without referencing velocity change.

Mark scheme (4 marks)

Velocity is a vector quantity, so it has both magnitude and direction.
As the object moves along a curved path, the direction of its velocity continuously changes.
A change in velocity (whether in magnitude or direction) constitutes acceleration.
Therefore, even though speed is constant, a change in the direction of velocity means a non-zero acceleration exists.

Key terms in this question

speed · curved path