| English | Principle of conservation of momentum |
|---|---|
| Bahasa Melayu | Prinsip keabadian momentum |
| 中文 | 动量守恒定律 |
Definition
The principle that the total momentum of a system remains constant in the absence of an external force, so total momentum before a collision equals total momentum after.
In exam terms
The principle of conservation of momentum states that when no external force acts on a system, its total momentum stays constant. In a collision or explosion this means the total momentum of the objects before the event equals the total momentum after it.
Momentum is a vector measured in kg m s⁻¹, so direction matters: take one direction as positive and the opposite as negative. For two objects the working is written as m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂.
For example, a 2 kg trolley at 3 m s⁻¹ striking a stationary 1 kg trolley gives total momentum before = 2 kg × 3 m s⁻¹ + 0 = 6 kg m s⁻¹, which the two must share after the collision.
Do not confuse it with…
Do not confuse conservation of momentum with conservation of kinetic energy. Momentum is conserved in every collision where no external force acts, but kinetic energy is only conserved in a perfectly elastic collision.
In an inelastic collision (objects sticking together) momentum is still conserved while some kinetic energy is changed into heat and sound.
In Paper 2 this is usually asked as state the principle of conservation of momentum, or calculate the common velocity after collision. Always show the before and after momentum expressions and keep a consistent positive direction, otherwise the sign of the answer will be wrong.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)