Not given in the exam, recall it.
What it is for
Newton's second law 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 |
|---|---|---|
| F | resultant force | N |
| m | mass | kg |
| a | acceleration | m s⁻² |
Rearrangements
m = F / aa = F / m
Worked example
Common trap
Understanding Newton's second law
Newton's second law in its common form is F = ma. Here F is the resultant (net) force in N, m is the mass in kg, and a is the acceleration in m s⁻².
The law says the acceleration of an object is directly proportional to the resultant force on it and inversely proportional to its mass. Push harder and it accelerates more; make it heavier and the same force gives less acceleration.
- To find the mass: m = F / a
- To find the acceleration: a = F / m
One newton is the force that gives a 1 kg mass an acceleration of 1 m s⁻², so 1 N = 1 kg m s⁻². The acceleration is always in the same direction as the resultant force.
A worked example, step by step
A resultant force acts on a car of mass m = 1200 kg, giving it an acceleration a = 2.5 m s⁻². Find the resultant force.
Write the formula, then substitute with units:
- F = ma
- F = 1200 kg × 2.5 m s⁻²
- F = 3000 kg m s⁻²
So F = 3.0 × 10³ N.
Because 1 N = 1 kg m s⁻², the unit kg m s⁻² is the same as the newton, which is why the answer is quoted in N. It is written in standard form to two significant figures, matching the precision of the data.
Common mistakes and how this is tested
The central trap is that F must be the resultant (net) force, not a single applied force. If friction or weight also acts, subtract them first to get the net force.
Other errors:
- Using weight in newtons where mass in kilograms is needed, or the reverse.
- Forgetting that acceleration is in the same direction as the resultant force.
- Leaving mass in grams instead of kilograms.
It is tested under state Newton's second law, calculate the resultant force or acceleration, and explain why a larger mass accelerates less for the same force. Show the resultant force clearly before substituting.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)