Who this is for: Cambridge IGCSE Physics (0625) students who want Momentum — calculating p = mv, applying conservation in collisions and linking impulse to force — to become a reliable source of marks instead of a formula they use without a clear before-and-after picture.
What query it owns: how to understand and revise Momentum in Cambridge IGCSE Physics.
Why this is safe: this page owns the Momentum revision-guide angle, while Tutopiya’s Momentum subtopic page owns the learning resource and the free Momentum quiz owns the practice.

Momentum appears regularly in Cambridge IGCSE Physics (0625) Extended papers. Examiners expect you to calculate momentum, apply conservation of momentum in closed collisions, and understand that a force acting for a time changes momentum (impulse). This guide explains the core formulas, the collision method, the question types that actually appear, and where to practise each skill.

Key takeaways

• Momentum p = mv — unit kg m/s; a vector with direction matching velocity.
• Conservation of momentum: total momentum before = total momentum after (in a closed system with no external resultant force).
• Impulse = FΔt = change in momentum Δp.
• In collisions, treat direction: choose positive and keep signs consistent.

What is Momentum in Cambridge IGCSE Physics?

Momentum measures how hard it is to stop a moving object. It depends on both mass and velocity: p = mv. In an isolated system, momentum is conserved — what one object loses, another gains. Impulse links force and time to momentum change: FΔt = Δp. Cambridge IGCSE questions often involve two objects colliding or one object splitting, where you equate total momentum before and after.

You can read the full explanation, worked examples and notes on Tutopiya’s Momentum subtopic page before you attempt questions.

The core ideas you must master

These four ideas appear again and again. Learn what each one means and the exam phrasing that signals it.

Idea	What it means	How the exam uses it
Momentum	p = mv; kg m/s	”Calculate the momentum of the truck”
Conservation	Σp before = Σp after	”Find the velocity after collision”
Impulse	FΔt = Δp	”Calculate the force exerted during impact”
Vector direction	Opposite velocities need opposite signs	”moving to the left”, “rebounds”

How to solve a collision question — step by step

The safest method works for sticking together, bouncing apart and explosion-type questions.

1. Write momentum before — add m₁v₁ + m₂v₂ with directions as signs.
2. Write momentum after — same format for final velocities (unknowns allowed).
3. Equate total before = total after.
4. Solve for the unknown velocity.
5. Check direction — does the sign match the physical situation?

Once you have worked through a few, test yourself with the free Momentum quiz — it tells you fast whether the method has actually stuck.

Elastic vs inelastic: what does the question tell you?

Students lose marks by ignoring direction or assuming kinetic energy is conserved when only momentum is required.

Situation	What to do	Typical signal words
Objects stick together	One final velocity for combined mass	”join”, “stick together”, “coalesce”
Objects bounce apart	Two final velocities; conservation of p	”rebounds”, “separate after collision”
Explosion / recoil	Total p before = 0 if initially at rest	”gun fires bullet”, “rocket”
Impulse question	Use FΔt = Δp	”force during impact”, “contact time”

Momentum in past-paper wording: command words that matter

Most lost marks come from sign errors and forgetting that momentum is a vector quantity.

Command word / phrase	What the question wants	Typical Momentum stem
Calculate / Work out	p = mv or conservation equation	”Work out the momentum of the ball.”
Determine	Find unknown velocity after collision	”Determine the speed of the car after impact.”
Explain	Link force, time and momentum change	”Explain how airbags reduce injury.”
State	Conservation principle	”State the law of conservation of momentum.”
Show that	Prove a given result with working	”Show that the final speed is 3 m/s.”

Worked exam-style stems (how to answer the wording)

Practising the wording — not just the formula — is what method marks reward. Here is how three real-style stems are answered.

1. “A 2 kg ball travels at 5 m/s. Calculate its momentum.” p = mv = 2 × 5 = 10 kg m/s in the direction of motion. Mark-scheme reward: formula, answer, unit.
2. “A 1000 kg car travelling at 10 m/s collides with a stationary 1000 kg car. They stick together. Find their common speed.” Before: 1000 × 10 + 0 = 10 000. After: 2000 × v. So v = 10 000/2000 = 5 m/s. Reward: conservation equation written clearly.
3. “A force of 500 N acts for 0.02 s on a ball. Calculate the change in momentum.” Δp = FΔt = 500 × 0.02 = 10 kg m/s. Reward: correct use of impulse formula.

When you can recognise the wording instantly, work the full set on the Motion, Forces and Energy topical past-paper questions and the Momentum quiz to lock the method in.

How Momentum connects to the rest of Motion, Forces and Energy

Momentum builds on Forces and Motion, and links to kinetic energy in Energy, Work and Power. Mass from Mass and Weight is essential in every calculation. When you are ready to move on, the Cambridge IGCSE Physics resource hub lets you jump straight from a weak subtopic into the next.

Common mistakes students make

• Adding momenta without considering direction (sign errors).
• Assuming kinetic energy is conserved in all collisions — only momentum is always conserved in closed systems.
• Using velocity in km/h without converting to m/s.
• Forgetting to add masses when objects stick together.
• Confusing momentum with force.

When you need more support

If Momentum questions keep tripping you up — especially conservation with opposite directions — work through the Motion, Forces and Energy topical past-paper questions and the Momentum quiz to pinpoint the exact gap, then get focused help from a Cambridge IGCSE Physics tutor to fix it quickly.

Frequently asked questions

Is Momentum hard in Cambridge IGCSE Physics? The maths is straightforward. Marks are lost on direction signs, unit conversion and confusing momentum with energy.

What is the unit of momentum? kg m/s (kilogram metres per second).

When is momentum conserved? In a closed system with no external resultant force — for example, two colliding trolleys on a frictionless track.

How do I revise Momentum effectively? Read the subtopic notes, practise conservation with signed velocities, then take the Momentum quiz. Revisit any collision questions you got wrong before moving on.

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