# 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

> 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.

## 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

- [constant velocity](https://www.gradenine.co.uk/glossary/constant-velocity)
- [momentum](https://www.gradenine.co.uk/glossary/momentum)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-car-is-travelling-at-a-a2af0a33) · Published by Druglandscape Ltd.