A car is travelling at a constant velocity along a straight road. The driver then applies the brakes, and the car decelerates to a stop. Explain why the car travels at constant velocity before braking, and describe what happens to the momentum of the car as it decelerates, including the cause of the change in momentum.

Edexcel A-Level Physics (9PH0) — 2.2 Forces, Newton's laws and momentum · Explain · 5 marks · View as Markdown

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

A car of mass 1200 kg is travelling at a constant velocity of 15 m/s along a straight, level road. The driver applies the brakes, and a resultant braking force acts on the car until it stops.

Model answer (5 marks)

At constant velocity the resultant force on the car is zero.
Newton’s First Law states that an object continues at constant velocity if the resultant force is zero.
The car has momentum because it has mass and velocity (p = mv).
As the car decelerates, its velocity decreases, so its momentum decreases.
The change in momentum is caused by the resultant braking force acting on the car (force causes change in momentum, Newton’s Second Law).

Examiner tips

  • State that the car’s speed is constant because the net force is zero, citing Newton’s First Law.
  • Explain that momentum is mv and that a decrease in v reduces p.
  • Mention that the braking force provides the net force that changes momentum, linking to Newton’s Second Law.
  • Use the exact wording from the mark scheme to secure full marks.

Common mistakes

  • Failing to mention that the net force is zero before braking.
  • Confusing acceleration with velocity when describing momentum change.
  • Not linking the braking force to the change in momentum via Newton’s Second Law.

Mark scheme (5 marks)

  1. At constant velocity the resultant force on the car is zero
  2. Newton's First Law states that an object continues at constant velocity if the resultant force is zero (or equivalent correct statement of Newton's First Law)
  3. The car has momentum because it has mass and velocity (momentum = mass × velocity)
  4. As the car decelerates, its velocity decreases, so its momentum decreases
  5. The change in momentum is caused by the resultant braking force acting on the car (force causes change in momentum / Newton's Second Law)

Key terms in this question

constant velocity · momentum

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