Basic Physics for Job Tests: Units, Motion, Light, Sound and Heat

Physics questions appear in almost every Everyday Science paper for police, FIA, PSC, NTS and teaching tests. The questions are usually short and direct, so a clear revision of units, motion, light, sound and heat can win you several easy marks. This note collects the most repeated facts in one place.

1. SI Units You Must Know

Quantity	SI unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Temperature	kelvin	K
Force	newton	N
Energy / work	joule	J
Power	watt	W
Pressure	pascal	Pa
Frequency	hertz	Hz
Electric charge	coulomb	C
Electric potential	volt	V
Resistance	ohm	Ω

• Speed, velocity and acceleration are measured in m/s and m/s².
• Speed has only magnitude (scalar); velocity has magnitude and direction (vector).
• Work = force × distance. Power = work ÷ time.
• 1 horsepower is about 746 watts.

2. Newton's Laws of Motion

• First law (inertia): a body stays at rest or in uniform motion unless an outside force acts on it. This is why passengers lurch forward when a bus brakes suddenly.
• Second law: force = mass × acceleration (F = ma).
• Third law: every action has an equal and opposite reaction. A rocket moves up because hot gases are pushed down.

Gravity

• The acceleration due to gravity near the Earth's surface is about 9.8 m/s² (often rounded to 10 m/s² in problems).
• Mass stays the same everywhere; weight changes with gravity. A body weighs less on the Moon because the Moon's gravity is weaker than the Earth's.
• Weight = mass × g.

3. Light

• Light travels in a vacuum at about 3 × 10⁸ m/s (300,000 km/s).
• Light from the Sun takes roughly 8 minutes to reach the Earth.
• Reflection: the angle of incidence equals the angle of reflection.
• Refraction: bending of light when it passes from one medium to another, for example a pencil looking bent in a glass of water.
• A plane mirror forms a virtual image of the same size as the object.
• A concave mirror is used in torches, headlights and shaving mirrors. A convex mirror is used as a rear-view mirror because it gives a wider field of view.
• A convex lens converges light and is used to correct long-sightedness (hypermetropia). A concave lens diverges light and corrects short-sightedness (myopia).
• White light splits into seven colours (VIBGYOR) when it passes through a prism. A rainbow is formed by refraction, reflection and dispersion of sunlight in water drops.

4. Sound

• Sound is a mechanical wave and needs a medium. It cannot travel through a vacuum.
• Sound travels fastest in solids, slower in liquids and slowest in gases.
• In air at room temperature the speed of sound is about 340 m/s.
• Frequency is measured in hertz. The normal range of human hearing is about 20 Hz to 20,000 Hz.
• Sound below 20 Hz is called infrasound; above 20,000 Hz is called ultrasound.
• An echo is the reflection of sound. A minimum distance of about 17 metres between the speaker and the reflecting surface is needed to hear a clear echo.
• Pitch depends on frequency, while loudness depends on amplitude. Loudness is measured in decibels (dB).

5. Heat and Temperature

• Heat is a form of energy; temperature tells us how hot or cold a body is.
• Water freezes at 0 °C and boils at 100 °C (at normal sea-level pressure). On the Kelvin scale these are 273 K and 373 K.
• Conversion: K = °C + 273.
• Heat travels by three methods:
  • Conduction: through solids, for example a metal spoon in hot tea.
  • Convection: through liquids and gases, for example boiling water or sea breeze.
  • Radiation: without any medium, for example heat from the Sun.
• Metals such as copper and aluminium are good conductors; wood, plastic and air are poor conductors (insulators).
• Water has a high specific heat capacity, so it heats up and cools down slowly. This is why it is used in cooling systems.

6. Electricity and Magnetism

• Current flows from the positive to the negative terminal (conventional current); electrons actually move the opposite way.
• Ohm's law: V = I × R.
• In a series circuit the same current flows through all components; in a parallel circuit the voltage is the same across each branch. Household appliances are connected in parallel.
• A fuse protects a circuit by melting when the current is too high.
• A generator converts mechanical energy into electrical energy; an electric motor converts electrical energy into mechanical energy. A transformer changes the voltage of an alternating current.
• Like magnetic poles repel and unlike poles attract. The Earth itself behaves like a giant magnet.

7. Quick Revision Questions

1. What is the SI unit of force? Newton
2. Which mirror is used as a vehicle rear-view mirror? Convex mirror
3. Which defect of the eye is corrected by a concave lens? Short-sightedness (myopia)
4. In which medium does sound travel fastest: air, water or steel? Steel
5. Which method of heat transfer needs no medium? Radiation

Tips for the Test

• Learn units and their symbols first; they are asked most often.
• When a calculation is asked, write the formula, put in the numbers and check the unit.
• Do not confuse mass with weight, or speed with velocity.

Test yourself with our Everyday Science MCQs, try more topics in General Knowledge MCQs and keep your practice going with the Daily Quiz.