Physics: Measurement, Motion, Force and Electricity
Core concepts
Physics questions cover SI units and measurement, scalar and vector quantities, speed, velocity and acceleration, Newton's laws, work, power and energy, heat and temperature, sound, light, and basic electricity including Ohm's law and series–parallel circuits.
Common traps
- Mass versus weight: mass is in kilograms and constant; weight is a force in newtons and changes with gravity (W = mg).
- Speed versus velocity: velocity includes direction, so a body moving in a circle at constant speed still has changing velocity.
- Work: if displacement is zero, work is zero even when a large force is applied - for example, pushing a wall.
- Series and parallel: in series, current is the same and voltage divides; in parallel, voltage is the same and current divides.
- Unit prefixes: 1 kW = 1000 W, 1 kWh is energy, not power.
Rule of thumb
Write the formula, substitute values with units, then check the unit of the answer. A wrong unit almost always means a wrong choice of formula.
Worked example
A 60 W lamp runs for 5 hours. Energy = power × time = 60 W × 5 h = 300 Wh = 0.3 kWh. If electricity costs 8 taka per kWh, the cost is 0.3 × 8 = 2.4 taka. Note that hours are kept as hours to obtain kWh directly.