Momentum (Higher)
- Momentum is a property of moving objects: p = m v, where p is momentum (kg m/s), m is mass (kg) and v is velocity (m/s).
- Momentum is a vector: it has the same direction as the velocity.
Conservation of momentum (Higher)
- In a closed system, the total momentum before an event is equal to the total momentum after the event.
- This is called conservation of momentum. It applies to collisions and explosions.
- Example: a 2 kg trolley at 3 m/s hits a stationary 1 kg trolley and they stick together. Momentum before = 6 kg m/s, so their velocity after = 6 ÷ 3 = 2 m/s.
- In an explosion (such as a gun firing), the total momentum is zero before, so the two parts move apart with equal and opposite momentum.
Changes in momentum (Higher, separate sciences)
- When a force acts on a moving object, its momentum changes: F = m Δv ÷ Δt. Force equals the rate of change of momentum.
- Increasing the time taken for the momentum to change reduces the force.
- Safety features such as air bags, seat belts, crumple zones, gym crash mats, cycle helmets and cushioned playground surfaces all increase the impact time, reducing the force on the person.
Key terms
- Momentum
- Mass × velocity (p = mv), measured in kg m/s.
- Conservation of momentum
- In a closed system, total momentum before an event equals total momentum after.
- Closed system
- A system with no external forces acting on it.
- Collision
- When two objects hit each other.
- Rate of change of momentum
- How quickly momentum changes; equal to the resultant force.
- Crumple zone
- A part of a car designed to crumple in a crash, increasing the impact time.