Physics Fundamentals
Prelims physics is school physics asked as recall: units, a handful of laws, and the everyday phenomena those laws explain. Nothing needs derivation — but the wrong unit or the reversed law loses the mark.
Mental model
Physics questions are almost always 'which conserved quantity is doing the work?' Sort every problem into one of four buckets — motion (Newton's three laws), energy/work/power, waves & light (reflection, refraction, sound), and electricity/magnetism. Then check the UNIT of the answer: if the option's unit does not match the quantity asked, it is a distractor. Unit-matching alone kills two options in most MCQs.
Associations
A short trigger on the left, everything it pulls with it on the right.
Every phenomenon has one law behind it.
Mirage and optical fibre — total internal reflection. Blue sky and red sunset — scattering. Aeroplane lift and a spinning ball's curve — Bernoulli. A ship floating — Archimedes. Learn the pairing and a phenomenon question becomes a law question.
Radio, micro, infra, visible, ultra, X, gamma.
The electromagnetic spectrum in order of increasing frequency and decreasing wavelength. Any question about which radiation has more energy, or which is used for what, is answered by position in this sequence.
Things to remember
The night-before list. Short enough to recall cold.
- 1SI base units: metre, kilogram, second, ampere, kelvin, mole and candela.
- 2Newton's first law defines inertia, the second gives force as rate of change of momentum, and the third gives action and reaction.
- 3Light travels at about three hundred thousand kilometres per second in vacuum; sound needs a medium and travels far slower.
- 4Ohm's law states that current is directly proportional to potential difference at constant temperature.
- 5Water boils below 100 degrees Celsius at high altitude because atmospheric pressure is lower; a pressure cooker uses the reverse effect.
- 6A concave mirror can form a magnified image and is used in headlights and shaving mirrors; a convex mirror always forms a diminished virtual image and is used as a rear-view mirror.
In detail
- SI units to recall cold: force = newton (N), work/energy = joule (J), power = watt (W), pressure = pascal (Pa), frequency = hertz (Hz).
- Newton's second law: force = mass x acceleration. The third law pairs (action-reaction) act on TWO DIFFERENT bodies, which is why they never cancel each other.
- Speed of light in vacuum is about 3 x 10^8 metres per second; speed of sound in air at room temperature is about 340 metres per second. Sound needs a medium, light does not.
- Ohm's law: V = I x R. Household appliances are wired in PARALLEL so each gets full voltage and one failure does not break the circuit.
- A rainbow is formed by refraction, total internal reflection and dispersion of sunlight inside water droplets — red bends least, violet bends most.
Where people slip
- Sound cannot travel through vacuum; light can. Options reversing this are wrong.
Check yourself
Nothing here is scored — it will not change your mastery figures.
Optical fibres transmit light over long distances mainly because of:
From the question bank
What is the SI unit of force?
From the question bank
What is the SI unit of power?
Books to read
- NCERT Class IX — Science
- NCERT Class X — Science
- Lucent's General Knowledge
Official sources
Hand-checked official domains only — the notes above carry no links of their own.