Forces
Lesson overview
This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.
What you will learn
Safety scenarios supplied on this page
Use the car, bicycle and collision examples to practise stopping-distance reasoning and momentum calculations.
Stopping distances and momentum infographic
Clear explanation
Stopping distance is thinking distance plus braking distance. Thinking distance depends on reaction time and speed. Braking distance depends on speed, road conditions, tyres, brakes and mass.
Momentum is mass times velocity. In a closed system, total momentum before a collision equals total momentum after the collision.
Safety features increase the time taken to stop during a collision. For the same change in momentum, a longer stopping time means a smaller average force.
Worked examples
Calculating momentum
A 70 kg cyclist moves at 6 m/s.
momentum = mass x velocity
momentum = 70 x 6 = 420
Quick checks
Choose an answer, then check your thinking.
1. What two parts make stopping distance?
2. A 2 kg object moves at 4 m/s. What is its momentum?
Practice questions
Question 1
A car has thinking distance 12 m and braking distance 28 m. Calculate stopping distance.
Reveal answer and marking guidance
Answer: 40 m.
Marking: Credit adding thinking and braking distances.
Question 2
A 1200 kg car travels at 15 m/s. Calculate momentum.
Reveal answer and marking guidance
Answer: 18 000 kg m/s.
Marking: Credit p = m v and 1200 x 15 = 18 000 kg m/s.
Question 3
Name two factors that increase braking distance.
Reveal answer and marking guidance
Answer: Greater speed and wet or icy roads.
Marking: Credit any two valid factors such as worn tyres, poor brakes, downhill slope or greater mass.
Question 4
Why does an airbag reduce injury risk?
Reveal answer and marking guidance
Answer: It increases the time over which the passenger slows down, reducing the average force.
Marking: Credit longer collision time and reduced force.
Practice ladder
Answers and marking guidance
The exact practice answers are hidden under each question so you can try first. For this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.
Common mistakes
- Forgetting thinking distance in total stopping distance.
- Saying momentum is the same as force.
- Ignoring velocity direction in momentum questions.
- Claiming safety features reduce momentum change rather than increasing stopping time.
Extension challenge
Compare two drivers at different speeds and explain why doubling speed more than doubles the danger.
Reveal answer
Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.
Exam-board guidance
Short board notes only. Learn the core physics above first.
AQA GCSE Physics
AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for stopping distances, momentum and safety.
OCR GCSE Physics
OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.
Pearson Edexcel GCSE Physics
Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.
Eduqas GCSE Physics
Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.
WJEC Wales
WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.
CCEA GCSE Physics
CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.
Next lesson
Next, continue with Wave Behaviour and Lenses.