Not given in the exam, recall it.
What it is for
Momentum is used in Force and Motion I (SPM Physics Form 4). Keep the SI units in every line of working. Force and Motion I
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| p | momentum | kg m s⁻¹ |
| m | mass | kg |
| v | velocity | m s⁻¹ |
Rearrangements
m = p / vv = p / m
Worked example
Common trap
Understanding momentum
Momentum is defined as the product of mass and velocity: p = mv. Here p is the momentum in kg m s⁻¹, m is the mass in kg, and v is the velocity in m s⁻¹.
Momentum measures how hard it is to stop a moving object. A heavy object or a fast object both carry a lot of momentum, because the two factors multiply together.
- To find the mass: m = p / v
- To find the velocity: v = p / m
Because velocity is a vector, momentum is also a vector: it has both size and direction. This matters in collision problems, where momentum in opposite directions is given opposite signs before adding.
A worked example, step by step
Find the momentum of a car of mass m = 1500 kg moving at velocity v = 12 m s⁻¹.
Write the formula, then substitute with units:
- p = mv
- p = 1500 kg × 12 m s⁻¹
- p = 18000 kg m s⁻¹
So p = 1.8 × 10⁴ kg m s⁻¹ in the direction of motion.
The unit comes straight from the multiplication: kg × m s⁻¹ = kg m s⁻¹. The answer is written in standard form to two significant figures, and the direction is stated because momentum is a vector.
Common mistakes and how this is tested
The main trap is treating momentum as a scalar and dropping the direction. Momentum is a vector, so include direction, and its unit is kg m s⁻¹, not N.
Other errors:
- Confusing momentum with kinetic energy, which uses v² not v.
- Forgetting to convert grams to kilograms or km h⁻¹ to m s⁻¹.
- Adding momenta without accounting for opposite directions in a collision.
It is tested under define momentum (mass × velocity), calculate the momentum of an object, and state the principle of conservation of momentum. Always give both the number and the unit kg m s⁻¹.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)