Impulse

Overview

Impulse describes the effect of a force acting over a time interval. It is especially useful in collisions because the force may be large and may change during the contact time.

What You Need to Know

• Force is the rate of change of momentum.
• Impulse is equal to change in momentum.
• For a constant force, impulse = force x time.
• For a varying force, impulse is the area under a force-time graph.
• If the same momentum change happens over a longer time, the average force is smaller.

How to Work Through It

1. Start by calculating momentum changes using final momentum minus initial momentum.
2. Link the momentum change to impulse.
3. Use rectangular and triangular areas on force-time graphs.
4. Apply the idea to safety examples such as padding, crumple zones, or bending knees when landing.

Check Your Understanding

• What are the units of impulse?
• Why is impulse a vector quantity?
• How do you find impulse from a force-time graph?
• Why does a longer stopping time reduce the average force?

Common Mistakes

• Treating impulse as a force instead of a change in momentum.
• Forgetting to include direction when calculating a momentum change.
• Using peak force when a question asks for average force.
• Finding graph gradient instead of area under a force-time graph.

Next Steps

• Practise force-time graph areas.
• Use impulse to strengthen explanations in collision and safety questions.