Lesson Plan

• Describe the definition of force as the rate of change of momentum.
• Explain how impulse relates to a change in momentum.
• Apply the impulse‑momentum theorem to solve quantitative problems for constant‑mass systems.
• Derive and use the variable‑mass form of F = dp/dt for situations such as rockets.
• Check solutions by verifying units and sign conventions.

• Projector and screen
• Whiteboard and markers
• Physics textbook (A‑Level Momentum chapter)
• Worksheets with impulse‑momentum problems
• Digital force sensor or spring‑scale demo kit
• Calculators
• PhET simulation on impulse and collisions (optional)

Begin with a short video of a car crash to highlight how forces act over very short times. Ask students what they already know about momentum and how it changes. State that by the end of the lesson they will be able to define force as dp/dt, use impulse to calculate momentum changes, and apply these ideas to both constant‑mass and variable‑mass situations.

1. Do‑now (5'): Quick quiz on the definition of momentum and its vector nature.
2. Mini‑lecture (10'): Derive F = dp/dt, show the constant‑mass reduction to F = ma, and introduce the impulse‑momentum theorem.
3. Demo (8'): Use a force sensor with a moving cart to illustrate impulse and measure Δp.
4. Guided practice (12'): In pairs, work through the provided collision example, filling in each step on the worksheet.
5. Variable‑mass discussion (7'): Explain the additional v dm/dt term with a rocket illustration.
6. Individual problem (8'): Solve a short rocket‑thrust problem applying the variable‑mass equation.
7. Exit ticket (5'): Students answer two short questions summarising the key concepts.

Recap the link between force, impulse, and momentum change, emphasizing the two forms of F = dp/dt. Collect exit tickets to gauge understanding, and assign homework: complete the remaining worksheet problems on impulse‑momentum and a short essay on how rockets use the variable‑mass term.