Physics (រូបវិទ្យា)
What the paper tests
Grade 9 physics covers motion, force and pressure, work and energy, simple electric circuits, magnetism, and basic optics. The exam rewards correct formulas, correct units, and clear reasoning about physical situations.
Core rules and concepts
- Speed: v = d/t; convert centimetres to metres and minutes to seconds first.
- Force is measured in newtons (N); weight is W = mg, where g ≈ 9.8 N/kg near Earth’s surface.
- Ohm’s law: V = IR, so I = V/R.
- In series, resistance adds: R = R₁ + R₂. In parallel, the total resistance is smaller than the smallest branch.
- Energy is conserved but can transfer between stores; useful output is always less than input because of heating and friction.
Worked example
A 12 V battery is connected to a 4 Ω resistor. Find the current. Reasoning: use I = V/R = 12/4 = 3 A. If a second 2 Ω resistor is added in series, total resistance is 4 + 2 = 6 Ω, so I = 12/6 = 2 A. The current falls because the total resistance increases. This shows the difference between one resistor and a series circuit.
Traps that cost marks
Do not mix milliamperes and amperes: 500 mA = 0.5 A. Do not confuse mass (kg) with weight (N). In optics, measure angles from the normal, not from the mirror surface. In circuit diagrams, a voltmeter goes in parallel and an ammeter in series.
Exam focus
Write the formula, substitute values with units, then state the answer with its unit.