A car travels along a straight road. It starts from rest and accelerates uniformly for 8 seconds, reaching a speed of 20 m/s. It then travels at a constant speed of 20 m/s for 12 seconds. Finally, the driver applies the brakes and the car decelerates uniformly to rest in 4 seconds. Describe the motion of the car during each of the three stages and explain what the velocity–time graph for the entire journey would look like.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car travels along a straight road. It starts from rest and accelerates uniformly for 8 seconds, reaching a speed of 20 m/s. It then travels at a constant speed of 20 m/s for 12 seconds. Finally, the driver applies the brakes and the car decelerates uniformly to rest in 4 seconds.
Model answer (5 marks)
Stage 1: The car starts from rest and accelerates uniformly; its speed rises steadily from 0 to 20 m s⁻¹ over 8 s.
Stage 2: The car moves at a constant speed of 20 m s⁻¹ for 12 s; there is no acceleration and the net force is zero.
Stage 3: The driver brakes and the car decelerates uniformly; its speed falls steadily from 20 m s⁻¹ to 0 over 4 s.
Velocity–time graph: a straight line with a positive slope from (0,0) to (8,20); a horizontal line at 20 m s⁻¹ from t = 8 s to t = 20 s; a straight line with a negative slope from (20,20) to (24,0).
Stage 2: The car moves at a constant speed of 20 m s⁻¹ for 12 s; there is no acceleration and the net force is zero.
Stage 3: The driver brakes and the car decelerates uniformly; its speed falls steadily from 20 m s⁻¹ to 0 over 4 s.
Velocity–time graph: a straight line with a positive slope from (0,0) to (8,20); a horizontal line at 20 m s⁻¹ from t = 8 s to t = 20 s; a straight line with a negative slope from (20,20) to (24,0).
Examiner tips
- Use the word ‘uniformly’ to show constant acceleration/deceleration.
- Show the three distinct line segments on the graph and label the axes.
- Mention the net force is zero during the constant‑speed stage.
Common mistakes
- Writing a curved line instead of straight lines for uniform motion.
- Forgetting to start the graph at the origin.
- Not stating that the slope of the deceleration line is negative.
Mark scheme (5 marks)
- Stage 1: the car accelerates uniformly / the velocity increases at a constant rate from 0 to 20 m/s
- Stage 1 on the graph: a straight line with a positive gradient (sloping upward) from the origin
- Stage 2: the car travels at constant velocity / no acceleration / resultant force is zero
- Stage 2 on the graph: a horizontal straight line at 20 m/s
- Stage 3: the car decelerates uniformly / velocity decreases at a constant rate back to zero, shown as a straight line with a negative gradient ending at zero velocity
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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