IGCSE Physics: Momentum Practice Questions
Momentum is mass multiplied by velocity, measured in kg m/s. In any collision or explosion with no external force, the total momentum before the event equals the total momentum after it.
Sub-topic 1.6 of Cambridge IGCSE Physics 0625 appears on the Extended papers and is almost entirely calculation. Because momentum is a vector, the direction convention you choose at the start decides whether the whole answer works. The questions below make you set that convention every time.
What you need to know for Momentum
- Momentump = mv, mass in kilograms multiplied by velocity in metres per second. The unit is kg m/s. Momentum is a vector, so direction matters.
- Conservation of momentumIn a closed system with no external resultant force, total momentum before an event equals total momentum after it. This applies to collisions and explosions alike.
- Sign conventionChoose one direction as positive before you start. Any velocity in the opposite direction is then negative, and the signs handle the vector nature automatically.
- ImpulseImpulse = force multiplied by time, and it equals the change in momentum. Written F x t = mv minus mu.
- Force as rate of change of momentumF = change in momentum divided by time taken. A longer collision time for the same change in momentum means a smaller force.
- Safety applicationsCrumple zones, airbags and seat belts all increase the time over which a passenger's momentum falls to zero, which reduces the force acting on them.
IGCSE Physics Momentum questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A trolley of mass 2.0 kg moving at 3.0 m/s collides with a stationary trolley of mass 4.0 kg. They stick together. Calculate their common velocity after the collision.
Show the worked answer
- Take the direction of the moving trolley as positive.
- Total momentum before = (2.0 x 3.0) + (4.0 x 0) = 6.0 kg m/s.
- After the collision the two trolleys move together, so the combined mass is 2.0 + 4.0 = 6.0 kg.
- Momentum is conserved, so 6.0 = 6.0 x v, giving v = 1.0 m/s in the original direction.
A ball of mass 0.15 kg travelling at 12 m/s hits a wall and rebounds at 8.0 m/s in the opposite direction. Calculate the change in momentum of the ball.
Show the worked answer
- Take the direction towards the wall as positive, so the rebound velocity is negative.
- Momentum before = 0.15 x 12 = 1.8 kg m/s.
- Momentum after = 0.15 x (minus 8.0) = minus 1.2 kg m/s.
- Change in momentum = final minus initial = minus 1.2 minus 1.8 = minus 3.0 kg m/s. The magnitude is 3.0 kg m/s, directed away from the wall.
Explain how a car's crumple zone reduces the force on the driver during a crash.
Show the worked answer
- During a crash the driver's momentum falls to zero. That change in momentum is fixed by the mass and the speed before impact.
- Force equals change in momentum divided by the time taken for that change.
- A crumple zone deforms progressively rather than stopping instantly, so the car and the driver take longer to come to rest.
- Since the change in momentum is the same but the time is longer, the force acting on the driver is smaller, so injuries are less severe. Airbags and seat belts work on the same principle.
A stationary firework of mass 0.80 kg explodes into two pieces. One piece of mass 0.30 kg moves east at 20 m/s. Calculate the velocity of the other piece.
Show the worked answer
- Take east as positive. Total momentum before the explosion is zero, because the firework was stationary.
- Momentum is conserved, so total momentum after must also be zero.
- Momentum of the first piece = 0.30 x 20 = 6.0 kg m/s east.
- The second piece has mass 0.80 minus 0.30 = 0.50 kg, and must carry minus 6.0 kg m/s. So v = minus 6.0 divided by 0.50 = minus 12 m/s, meaning 12 m/s west.
Common mistakes in this topic
- Ignoring direction and treating momentum as a scalar.
- Forgetting to combine masses when objects stick together.
- Using speed rather than velocity when an object reverses direction.
- Omitting the unit kg m/s.
- Saying momentum is conserved in a collision where an external force such as friction acts significantly. State that the system is closed.
Exam tips
- Write your positive direction down before the first line of working. It prevents nearly every sign error in this topic.
- Set out conservation questions as total momentum before equals total momentum after, then substitute.
- In a rebound, the change in momentum is larger than you expect because the velocities have opposite signs.
- For safety questions, always name the quantity being extended: the time of the collision.
- Check the unit of your answer. Momentum is kg m/s, force is N, impulse is N s which is the same as kg m/s.
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Momentum FAQs
What is the principle of conservation of momentum?
In a closed system where no external resultant force acts, the total momentum before an event equals the total momentum after it. This applies to collisions, where objects come together, and to explosions, where an object separates into pieces.
How do I handle direction in momentum calculations?
Choose one direction as positive before starting, and treat any velocity in the opposite direction as negative. The signs then handle the vector nature of momentum automatically. State your convention in the working, because examiners award marks for a consistent method.
Why do airbags reduce injury?
The change in momentum a passenger experiences is fixed by their mass and the speed of the crash. Force equals change in momentum divided by time, so extending the time over which the passenger slows reduces the force acting on them. An airbag increases that time.
What is impulse?
Impulse is force multiplied by the time for which it acts, and it equals the change in momentum produced. Its unit, the newton second, is equivalent to kg m/s. This relationship is why a small force acting for a long time can produce the same change as a large force acting briefly.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.