A car is travelling along a straight road at a constant speed of 20 m/s. The driver then applies the brakes and the car slows down to a stop. Explain the difference between speed and velocity, and explain what is meant by the car having a negative acceleration as it slows down.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Speed is a scalar quantity – it has magnitude only and tells how fast an object is moving.
Velocity is a vector quantity – it has magnitude and direction; it is the speed in a stated direction.
Acceleration is the rate of change of velocity.
Negative acceleration (deceleration) means the velocity is decreasing – the change in velocity is opposite to the direction of motion, so the car slows down.
Velocity is a vector quantity – it has magnitude and direction; it is the speed in a stated direction.
Acceleration is the rate of change of velocity.
Negative acceleration (deceleration) means the velocity is decreasing – the change in velocity is opposite to the direction of motion, so the car slows down.
Examiner tips
- Use the exact terms ‘scalar’ and ‘vector’ when describing speed and velocity.
- Show the relationship ‘acceleration = change in velocity / time’ to justify negative acceleration.
- Explain that negative acceleration is simply a reduction in speed, not a ‘negative speed’.
- Keep each point brief and directly linked to the marks listed.
Common mistakes
- Confusing speed with velocity or omitting the direction component.
- Using ‘negative speed’ instead of ‘negative acceleration’ to describe slowing down.
- Failing to state that acceleration is a vector and that negative acceleration is a change in velocity opposite to motion.
Mark scheme (4 marks)
- Speed is a scalar quantity — it has magnitude only / only tells you how fast an object is moving
- Velocity is a vector quantity — it has both magnitude and direction / velocity is speed in a stated direction
- Acceleration is the rate of change of velocity
- Negative acceleration (deceleration) means the velocity is decreasing / the car is slowing down so the change in velocity is in the opposite direction to motion
Key terms in this question
speed · velocity · acceleration
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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