MYP Physics: Momentum (Extended) — Your Questions Answered

Momentum is a vector quantity — direction matters. A 2 kg trolley moving right at 3 m/s has momentum +6 kg·m/s; the same trolley moving left has -6 kg·m/s. Students who forget the sign lose marks on collision questions where objects move in opposite directions before impact.

Quick tip: always draw an arrow or write '+right, -left' at the start of a collision question. Examiners in MYP criterion C and D tasks specifically check whether direction was handled correctly, not just whether the number is right.

Worked example: a 1000 kg car slows from 20 m/s to 0 m/s in 0.5 s during a crash. Δp = m(v₂ - v₁) = 1000 × (0 - 20) = -20,000 kg·m/s. Impulse = -20,000 kg·m/s, so average force F = Δp/Δt = -20,000/0.5 = -40,000 N. Extend the collision time to 2 s (crumple zone) and F drops to -10,000 N — the same impulse, a much smaller force.

Steps to calculate momentum:

1. Identify mass (convert g → kg if needed).
2. Identify velocity, including direction.
3. Multiply: p = mv.
4. State the unit, kg·m/s, and the direction.

Example: a 500 g ball moving at 8 m/s right. Convert 500 g = 0.5 kg. p = 0.5 × 8 = 4 kg·m/s (right).

Worked example: trolley A (2 kg, 4 m/s) collides with stationary trolley B (2 kg, 0 m/s) and they stick together. Total momentum before = (2×4) + (2×0) = 8 kg·m/s. After collision, combined mass = 4 kg, so v = 8/4 = 2 m/s. Both trolleys move off together at 2 m/s.

Checklist before you start a collision question:

1. Draw a diagram with a chosen positive direction.
2. Write down p(before) = p(after).
3. Decide if objects stick together (use combined mass) or separate (keep masses separate).
4. Substitute known values, solve for the unknown velocity.
5. Check the sign of your answer makes physical sense.

Common mistake: students forget that a stationary object still has a momentum term (m × 0), which is fine to write but easy to accidentally drop from the equation entirely.

According to the MYP: Sciences guide, students are expected to apply scientific reasoning to interpret data and evaluate the validity of a method — a momentum investigation (e.g. trolley collisions on a track) is a classic vehicle for this because it's easy to measure mass and velocity but hard to eliminate friction, giving genuine scope for evaluation.

Quick tip: before submitting an answer, ask yourself — did I state a direction, are my units in kg and m/s, and is this genuinely a closed system? If a question mentions a wall, floor or the ground being involved in the collision, momentum is usually NOT conserved for the objects alone, because the Earth absorbs some of it.

A method that scores well against Criterion C typically includes: a controlled variable list, at least 3-5 repeat trials per condition, a percentage difference calculation between total momentum before and after, and a graph or table clearly labelled with units. Naming friction, air resistance, or reaction-time timing error as sources of uncertainty is far stronger than generic phrasing.

This links directly to the MYP Sciences aim of understanding the relationships between science and society. Engineers use impulse-force calculations (F = Δp/Δt) to design safety equipment, which is exactly the kind of real-world application examiners like to see referenced in Criterion D reflections.

In my experience teaching MYP 4-5, students who practise 8-10 mixed collision problems (some with objects sticking together, some bouncing apart) tend to stop making direction errors almost entirely — it's a skill that comes from repetition, not from re-reading the theory.

On RevisionPrep, MYP Physics Topical Worksheets group momentum questions by difficulty, so your child can start with single-object calculations before moving to two-object collisions — a structure that mirrors how the concept is actually taught and assessed.