A 60 kg astronaut travels to the Moon. On the lunar surface:

His mass stays 60 kg but his weight becomes one-sixth

Both his mass and weight become one-sixth

His mass stays 60 kg but his weight becomes one-sixth

His weight stays the same but his mass becomes one-sixth

Domestic appliances are connected in parallel because:

Each appliance gets the full voltage and works independently

The current through each is the same

Physics — Matter, Motion, Heat, Sound, Light, Electricity & Simple Machines

The GAT General Knowledge section is worth 400 marks — the single largest block in the whole NDA written exam — and physics reliably supplies the most questions inside it. The level is Class 9–10, not Class 12: you are asked what a phenomenon is and where you see it, not to derive anything. That makes it a memory-and-understanding paper rather than a numerical one, so the right preparation is to attach every law to a concrete example you cannot forget. This chapter does that throughout, and deliberately includes the defence and everyday applications the examiner favours.

Properties of Matter & Newton's Laws

Quantities that are constantly confused
Quantity	What it is	Unit
Mass	Amount of matter — the same everywhere, a scalar	kilogram (kg)
Weight	Force of gravity on that mass = mg — changes with place, a vector	newton (N)
Density	Mass per unit volume	kg/m³
Pressure	Force per unit area	pascal (Pa) = N/m²

🎯 Why you weigh less on the Moon but are not thinner

Lunar gravity is about 1/6 of Earth's, so a 60 kg person weighs 588 N on Earth and roughly 98 N on the Moon — but the mass stays 60 kg. This is the most-asked distinction in the whole physics section. A related favourite: a beam balance compares masses and so reads the same on the Moon, while a spring balance measures weight and reads one-sixth.

First law (inertia): a body continues at rest or in uniform motion unless a net external force acts. This is why you lurch forward when a bus brakes — your body keeps moving.
Second law: F = ma, or more generally force = rate of change of momentum. A cricketer draws his hands back while catching to increase the time and so reduce the force.
Third law: every action has an equal and opposite reaction. This is what makes a rocket and a recoiling rifle work — gases go one way, the body goes the other.

Gravitation. Newton's law: every two masses attract with F = Gm₁m₂/r². Acceleration due to gravity g ≈ 9.8 m/s² on Earth; it is maximum at the poles and minimum at the equator (the Earth bulges there, so r is larger), and it decreases both as you go up and as you go deep below the surface — at the centre of the Earth g is zero. Escape velocity from Earth is about 11.2 km/s.

Heat Transfer & Sound

Three modes of heat transfer
Mode	How	Example
Conduction	Through a solid, particle to particle, no bulk movement	A metal spoon heating in tea
Convection	The fluid itself moves, carrying heat	Boiling water; sea breeze; a room heater
Radiation	Electromagnetic waves — needs no medium	The Sun's heat reaching Earth through vacuum

🧠 Sound needs a medium; light does not

Sound is a mechanical, longitudinal wave, so it cannot travel through vacuum — an alarm bell in a vacuum jar goes silent while its light stays visible. Speed of sound in air ≈ 332–340 m/s, in water ≈ 1,500 m/s, in steel ≈ 5,000 m/s: sound travels fastest in solids because the particles are closest. Light, being electromagnetic, is the opposite — fastest in vacuum at 3 × 10⁸ m/s and slower in denser media.

Audible range for humans is 20 Hz to 20,000 Hz. Below is infrasonic, above is ultrasonic.
SONAR uses ultrasonic pulses to find submarines and measure ocean depth — a direct defence application.
Pitch depends on frequency, loudness on amplitude, and quality/timbre on waveform — three different properties, often mixed up.
An echo needs the reflector at least ~17 m away, so that the reflected sound arrives after 0.1 s.

Light, Electricity & Magnetism

Light. Reflection obeys "angle of incidence = angle of reflection". Refraction — bending on entering a new medium — explains why a stick looks bent in water and why stars twinkle. Dispersion through a prism splits white light into seven colours, VIBGYOR: violet bends most and red least, because violet has the shortest wavelength. A rainbow is dispersion plus internal reflection in raindrops.

Mirrors, lenses and where they are used
Device	Image	Use
Plane mirror	Virtual, same size, laterally inverted	Ordinary mirror; periscope
Concave mirror	Real or virtual, can magnify	Shaving mirror, headlight, solar cooker, dentist's mirror
Convex mirror	Always virtual, diminished, wide field	Vehicle rear-view mirror
Convex lens	Converging	Corrects hypermetropia (long sight)
Concave lens	Diverging	Corrects myopia (short sight)

🧠 Series or parallel — remember it by the house wiring

Ohm's law is V = IR. In series the current is the same everywhere and resistances add (R = R₁ + R₂), so one broken bulb kills the whole string — which is why old fairy lights all failed together. In parallel the voltage is the same across each branch and 1/R = 1/R₁ + 1/R₂, so each appliance works independently — which is exactly how a house is wired. A fuse is always in series so it can break the circuit, and the total resistance in parallel is always less than the smallest individual resistance.

Like magnetic poles repel, unlike attract. A freely suspended magnet points north–south — the basis of the compass.
A magnet's poles cannot be separated: break it and you get two complete magnets.
Electric current produces a magnetic field (Oersted); a moving magnet produces current (Faraday's induction) — the principle of the generator and the transformer.
Electricity is billed in kilowatt-hours — a unit of energy, not power.

Simple Machines — Levers & Pulleys

A machine does not create energy; it only makes work easier or more convenient, by changing the direction or the magnitude of the force. Mechanical advantage = load ÷ effort. Since some effort always goes into friction, real efficiency is never 100%.

The three classes of lever
Class	Order	Examples
First	Fulcrum between effort and load	Scissors, seesaw, crowbar, beam balance
Second	Load between fulcrum and effort	Wheelbarrow, nutcracker, bottle opener
Third	Effort between fulcrum and load	Tongs, forearm, fishing rod, broom

🧠 F-L-E: the order of the middle element names the class

Ask only one question — what is in the middle? Fulcrum in the middle → first class. Load in the middle → second class. Effort in the middle → third class. Only the third class has a mechanical advantage less than 1, meaning you apply more force than the load — you accept that to gain speed or reach, which is why your forearm and a fishing rod are built that way.

Pulleys. A single fixed pulley gives a mechanical advantage of 1 — it only changes the direction of your pull, which is why a flag is easy to hoist while standing on the ground. A single movable pulley gives an advantage of 2. A block and tackle with n rope segments supporting the load gives an advantage of about n, which is how a crane lifts a load far heavier than the puller.

Key takeaways

Mass is constant everywhere; weight = mg and changes with place. On the Moon weight is 1/6.
g is maximum at the poles, minimum at the equator, and zero at the Earth's centre.
Rocket propulsion and rifle recoil both demonstrate Newton's third law.
Radiation needs no medium; sound needs one, so it is silent in vacuum.
Sound is fastest in solids (steel ≈ 5,000 m/s); light is fastest in vacuum.
Convex mirror for rear-view; concave for headlights; concave LENS corrects myopia.
House wiring is parallel so appliances work independently; a fuse sits in series.
Lever class = what sits in the middle: fulcrum first, load second, effort third.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

A 60 kg astronaut travels to the Moon. On the lunar surface:
1. Both his mass and weight become one-sixth
2. His mass stays 60 kg but his weight becomes one-sixth
3. His weight stays the same but his mass becomes one-sixth
4. Neither changes
Show answer
Answer: B — His mass stays 60 kg but his weight becomes one-sixth
Mass is the quantity of matter and does not depend on location. Weight is mg, and lunar g is about one-sixth of Earth's, so only the weight falls.
The recoil of a rifle when fired is an example of:
1. Newton's first law
2. Newton's second law
3. Newton's third law
4. Law of gravitation
Show answer
Answer: C — Newton's third law
The bullet is pushed forward and the rifle is pushed back with an equal and opposite force — action and reaction, the third law. Rocket propulsion works the same way.
Heat from the Sun reaches the Earth mainly by:
1. Conduction
2. Convection
3. Radiation
4. Evaporation
Show answer
Answer: C — Radiation
Space is a vacuum, and only radiation can cross it since it needs no medium. Conduction and convection both require matter to carry the heat.
Sound travels fastest in:
1. Vacuum
2. Air
3. Water
4. Steel
Show answer
Answer: D — Steel
Sound needs particles to pass the vibration along, so it is fastest where particles are closest — in solids (steel ≈ 5,000 m/s versus water ≈ 1,500 and air ≈ 340). In vacuum it does not travel at all.
The mirror used as a rear-view mirror in vehicles is:
1. Plane
2. Concave
3. Convex
4. Cylindrical
Show answer
Answer: C — Convex
A convex mirror always forms a small, erect, virtual image and covers a wide field of view — exactly what a driver needs. A concave mirror is used in headlights and shaving mirrors instead.
Domestic appliances are connected in parallel because:
1. It saves electricity
2. Each appliance gets the full voltage and works independently
3. The total resistance increases
4. The current through each is the same
Show answer
Answer: B — Each appliance gets the full voltage and works independently
In parallel the voltage across each branch is the same and a break in one branch does not affect the others, so a fan keeps running when a bulb fails. Total resistance actually decreases, and equal current through each is a feature of series.
A pair of scissors is an example of which class of lever?
1. First class
2. Second class
3. Third class
4. Not a lever at all
Show answer
Answer: A — First class
The rivet (fulcrum) lies between the handles where effort is applied and the blades where the load is, so it is first class — like a seesaw or a crowbar. A wheelbarrow is second class and tongs are third.
SONAR, used to detect submarines, works on:
1. Infrasonic waves
2. Ultrasonic waves
3. Radio waves
4. X-rays
Show answer
Answer: B — Ultrasonic waves
SONAR sends ultrasonic pulses (above 20,000 Hz) and times their echo. Radio waves and X-rays are electromagnetic and are absorbed very quickly by water, which is why sound rather than radio is used underwater.