IB Physics: Momentum & Impulse FAQ

You need the definition p = mv, the impulse-momentum theorem (impulse = change in momentum = FΔt), conservation of momentum in isolated systems, and the difference between elastic and inelastic collisions. HL adds force-time graph analysis and combining momentum with energy conservation in two-dimensional collision problems.

Momentum (p = mv) is a property an object has at a single instant — mass times velocity, measured in kg m/s. Impulse is what changes that momentum over an interval: it equals force multiplied by the time the force acts, and it's numerically equal to the resulting change in momentum, Δp.

Start every collision question by writing the conservation of momentum equation: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. For elastic collisions, kinetic energy is also conserved, so you get a second equation. For inelastic collisions (including perfectly inelastic, where objects stick together), only momentum is conserved.

The area enclosed between a force-time graph and the time axis gives the impulse delivered, in newton-seconds (N s), which equals the change in momentum. For a curved graph, examiners expect you to count squares or use the trapezium rule rather than assume a straight line.

Momentum conservation reappears in Topic 7 (Nuclear and particle physics) when analysing particle collisions and decay, and momentum-energy ideas underpin explanations of rocket propulsion and safety features like airbags and crumple zones — both popular Paper 2 extended-response contexts.

The core definitions and one-dimensional collision problems are identical at SL and HL. HL students are additionally expected to handle two-dimensional collisions using vector components and to interpret more complex force-time graphs, often combined with energy calculations in the same question.

No — Paper 1 and Paper 2 are set separately for SL and HL, though momentum questions can look similar at first glance. HL papers are more likely to require two-dimensional vector momentum problems and to combine momentum with impulse-time graph analysis in a single multi-part question.

Get comfortable writing the momentum conservation equation from memory, then practise applying it to unfamiliar contexts — explosions, recoil, collisions on a frictionless track. Examiners reward students who state whether momentum, kinetic energy, or both are conserved before calculating, so make that your first written line every time.

The biggest one: forgetting momentum is a vector, so directions (and signs) matter — a ball bouncing back has negative velocity relative to its approach. Others include mixing up elastic and inelastic conditions, and forgetting units (kg m/s for momentum, N s for impulse, which are numerically equivalent).

You need p = mv, impulse = FΔt = Δp, and the conservation equation m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. For elastic collisions, also know that kinetic energy before equals kinetic energy after. These aren't all printed identically in the data booklet, so check it before your exam.

Momentum appears in Paper 1 (multiple choice, testing quick conceptual recall), Paper 2 (structured and extended-response questions often worth 6-9 marks combining calculation with explanation), and occasionally in Paper 3 as a data-based or experimental question about measuring collision velocities.

Yes — momentum and collisions are a genuinely popular Internal Assessment choice because they're easy to investigate with trolleys, light gates, and a track, and they naturally generate quantitative data suitable for the IA's exploration and analysis criteria.