Concept

Physical quantities are measured in units; the SI system defines seven base units. Matter exists mainly as solid, liquid and gas, distinguished by how tightly its particles are held.

Knowing the correct SI unit of each quantity is frequently tested.

Key Points & Formulas

• SI base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature).
• Derived units: force → newton, energy → joule, power → watt, pressure → pascal.
• Solids have fixed shape & volume; liquids fixed volume only; gases neither.
• Density = mass/volume (kg/m³).

Worked Example

Q. State the SI units of force, work and power.

Force = newton (N = kg·m/s²).

Work/energy = joule (J = N·m).

Power = watt (W = J/s).

Practice MCQs

Q1. The SI unit of pressure is the

(A) Pascal (B) Newton (C) Joule (D) Watt

Answer: A. Pressure = force/area; its SI unit is the pascal (N/m²).

Q2. Which state of matter has a fixed volume but no fixed shape?

(A) Liquid (B) Solid (C) Gas (D) Plasma

Answer: A. Liquids keep their volume but take the shape of the container.

Concept

Kinematics describes motion using displacement, velocity and acceleration. Velocity is the rate of change of displacement; acceleration is the rate of change of velocity.

The three equations of motion link these for uniform acceleration.

Key Points & Formulas

• Speed is scalar; velocity is a vector (has direction).
• Equations of motion: v = u + at; s = ut + ½at²; v² = u² + 2as.
• Uniform velocity ⇒ zero acceleration.
• Acceleration unit: m/s².

Worked Example

Q. A car starting from rest accelerates at 2 m/s² for 5 s. Find its final velocity.

u = 0, a = 2 m/s², t = 5 s.

v = u + at = 0 + 2×5 = 10 m/s.

Practice MCQs

Q1. A body moving with uniform velocity has acceleration

(A) Zero (B) Constant non-zero (C) Increasing (D) Negative

Answer: A. Uniform velocity means no change in velocity, so acceleration = 0.

Q2. The quantity that has both magnitude and direction is

(A) Velocity (B) Speed (C) Distance (D) Mass

Answer: A. Velocity is a vector; speed, distance and mass are scalars.

Concept

Newton's three laws govern motion. The first defines inertia, the second relates force to the rate of change of momentum (F = ma), and the third states that every action has an equal and opposite reaction.

Momentum = mass × velocity and is conserved in collisions.

Key Points & Formulas

• First law (inertia): a body stays at rest/uniform motion unless acted on by a force.
• Second law: F = ma = rate of change of momentum.
• Third law: action and reaction are equal and opposite.
• Momentum p = mv; conserved when no external force acts.

Worked Example

Q. Find the force needed to accelerate a 5 kg body at 3 m/s².

F = ma.

F = 5 × 3 = 15 N.

Practice MCQs

Q1. Newton's first law is also called the law of

(A) Inertia (B) Momentum (C) Gravitation (D) Acceleration

Answer: A. It describes inertia — resistance to change in motion.

Q2. The recoil of a gun on firing illustrates Newton's

(A) Third law (B) First law (C) Law of gravitation (D) Second law

Answer: A. Action (bullet forward) and reaction (gun back) are equal and opposite.

Concept

Work is done when a force moves an object; power is the rate of doing work; energy is the capacity to do work. Mechanical energy is the sum of kinetic and potential energy.

Energy is conserved — it changes form but is neither created nor destroyed.

Key Points & Formulas

• Work W = Force × displacement × cosθ (joule).
• Power P = Work/time (watt); 1 horsepower ≈ 746 W.
• Kinetic energy = ½mv²; potential energy = mgh.
• Law of conservation of energy: total energy stays constant.

Worked Example

Q. A 2 kg ball is raised to a height of 5 m (g = 10 m/s²). Find its potential energy.

PE = mgh.

= 2 × 10 × 5 = 100 J.

Practice MCQs

Q1. The kinetic energy of a body of mass 4 kg moving at 3 m/s is

(A) 18 J (B) 12 J (C) 36 J (D) 6 J

Answer: A. KE = ½mv² = ½·4·9 = 18 J.

Q2. The SI unit of power is the

(A) Watt (B) Joule (C) Newton (D) Pascal

Answer: A. Power is energy per unit time, measured in watts.

Concept

Every mass attracts every other mass with a force described by Newton's law of gravitation. Near the Earth this gives a body its weight and a free-fall acceleration g.

Mass is constant; weight depends on g and so varies with location.

Key Points & Formulas

• Gravitational force F = G m₁m₂ / r².
• Weight W = mg; g ≈ 9.8 m/s² near the Earth's surface.
• g is greatest at the poles, least at the equator; decreases with altitude.
• Weight on the Moon ≈ 1/6 of that on the Earth.

Worked Example

Q. A body has mass 10 kg. Find its weight on Earth (g = 9.8 m/s²).

W = mg.

= 10 × 9.8 = 98 N.

Practice MCQs

Q1. The weight of a body is maximum at the

(A) Poles (B) Equator (C) Centre of Earth (D) Top of a mountain

Answer: A. g is greatest at the poles, so weight is maximum there.

Q2. Compared with Earth, a body's mass on the Moon is

(A) The same (B) One-sixth (C) Six times (D) Zero

Answer: A. Mass is constant everywhere; only weight changes.

Concept

A body is in equilibrium when the net force and net turning effect on it are zero. The parallelogram law gives the resultant of two forces acting at a point.

Stable, unstable and neutral equilibrium depend on how the centre of gravity moves when disturbed.

Key Points & Formulas

• Equilibrium: net force = 0 and net moment (torque) = 0.
• Parallelogram law: resultant R = √(P² + Q² + 2PQ cosθ).
• Lower centre of gravity and wider base ⇒ greater stability.
• Stable: CG rises on tilting; unstable: CG falls; neutral: CG stays level.

Worked Example

Q. Two perpendicular forces of 3 N and 4 N act at a point. Find their resultant.

θ = 90°, so cosθ = 0.

R = √(3² + 4² + 0) = √25 = 5 N.

Practice MCQs

Q1. A body has the greatest stability when its centre of gravity is

(A) Low and base wide (B) High and base narrow (C) High and base wide (D) Low and base narrow

Answer: A. A low CG with a wide base resists toppling best.

Q2. For a body in equilibrium, the net force on it is

(A) Zero (B) Maximum (C) Constant non-zero (D) Variable

Answer: A. Equilibrium requires zero net force and zero net torque.

Concept

Density is mass per unit volume; specific gravity (relative density) compares a substance's density with that of water. Archimedes' principle explains buoyancy — why objects float or sink.

An object floats if its density is less than that of the fluid.

Key Points & Formulas

• Density = mass / volume.
• Specific gravity = density of substance / density of water (no unit).
• Archimedes: upthrust = weight of fluid displaced.
• Floats if density < fluid density; sinks if greater.

Worked Example

Q. A block of 200 g occupies 250 cm³. Find its density and state whether it floats in water.

Density = 200/250 = 0.8 g/cm³.

Water's density is 1 g/cm³; 0.8 < 1, so the block floats.

Practice MCQs

Q1. An iron nail sinks in water because its density is

(A) Greater than water (B) Less than water (C) Equal to water (D) Zero

Answer: A. Objects denser than the fluid sink.

Q2. The upthrust on a body in a liquid equals the

(A) Weight of liquid displaced (B) Weight of the body (C) Volume of the body (D) Density of liquid

Answer: A. Archimedes' principle: upthrust = weight of displaced fluid.

Concept

Pressure is force per unit area. Fluids exert pressure that increases with depth; atmospheric pressure is measured with a barometer.

The mercury barometer was devised by Torricelli.

Key Points & Formulas

• Pressure = force / area (pascal).
• Liquid pressure = hρg (depth × density × g); independent of area.
• Atmospheric pressure ≈ 76 cm of mercury at sea level.
• A barometer measures atmospheric pressure; a manometer measures gas pressure.

Worked Example

Q. Why does a sharp knife cut better than a blunt one?

A sharp edge has a very small contact area.

For the same force, pressure = force/area is much larger.

Higher pressure cuts more easily.

Practice MCQs

Q1. Atmospheric pressure is measured by a

(A) Barometer (B) Thermometer (C) Hygrometer (D) Ammeter

Answer: A. A barometer measures atmospheric pressure.

Q2. Liquid pressure at a point depends on

(A) Depth and density (B) Area of container (C) Shape of container (D) Mass of container

Answer: A. Pressure = hρg, depending on depth and density, not area or shape.

Concept

Heat is energy in transit due to a temperature difference; temperature measures the degree of hotness. Heat changes the state of matter and is transferred by conduction, convection and radiation.

Latent heat is the heat absorbed/released during a change of state at constant temperature.

Key Points & Formulas

• Modes of heat transfer: conduction (solids), convection (fluids), radiation (no medium).
• Temperature scales: Celsius, Fahrenheit, Kelvin (K = °C + 273).
• Latent heat: state change occurs at constant temperature.
• Water has an unusually high specific heat.

Worked Example

Q. Convert 27 °C to the Kelvin scale.

K = °C + 273.

= 27 + 273 = 300 K.

Practice MCQs

Q1. Heat transfer in a vacuum occurs by

(A) Radiation (B) Conduction (C) Convection (D) Diffusion

Answer: A. Radiation needs no medium, so it works in a vacuum (e.g., the Sun's heat).

Q2. The boiling point of water at sea level is

(A) 100 °C (B) 0 °C (C) 50 °C (D) 273 °C

Answer: A. Water boils at 100 °C at normal atmospheric pressure.

Concept

Sound is a longitudinal mechanical wave that needs a medium to travel. Its pitch depends on frequency and loudness on amplitude. Echoes arise from reflection of sound.

Sound travels fastest in solids and cannot travel through a vacuum.

Key Points & Formulas

• Sound speed: solids > liquids > gases; ≈ 343 m/s in air.
• Pitch ↔ frequency; loudness ↔ amplitude.
• Audible range for humans: 20 Hz to 20,000 Hz.
• Echo: reflection of sound from a distant surface.

Worked Example

Q. Why can sound not travel through a vacuum?

Sound is a mechanical wave that needs particles to vibrate.

A vacuum has no medium/particles.

Hence sound cannot propagate through it (unlike light).

Practice MCQs

Q1. The pitch of a sound depends on its

(A) Frequency (B) Amplitude (C) Speed (D) Loudness

Answer: A. Higher frequency gives higher pitch.

Q2. Sound travels fastest in

(A) Steel (B) Water (C) Air (D) Vacuum

Answer: A. Sound travels fastest in solids like steel and not at all in vacuum.

Concept

Light travels in straight lines and can be reflected (bouncing off a surface) or refracted (bending when passing between media). Mirrors and lenses form images used in many devices.

Concave mirrors and convex lenses can converge light; convex mirrors and concave lenses diverge it.

Key Points & Formulas

• Law of reflection: angle of incidence = angle of reflection.
• Refraction: light bends towards the normal entering a denser medium.
• Concave mirror: converging (used in headlights, shaving mirrors).
• Dispersion: a prism splits white light into seven colours.

Worked Example

Q. Which mirror is used as a vehicle's rear-view mirror and why?

A convex mirror is used.

It always forms an erect, diminished image and gives a wider field of view.

This lets the driver see more of the traffic behind.

Practice MCQs

Q1. The splitting of white light by a prism is called

(A) Dispersion (B) Reflection (C) Diffraction (D) Absorption

Answer: A. A prism disperses white light into its component colours.

Q2. A concave mirror is used in a torch because it

(A) Produces a parallel beam (B) Diverges light (C) Forms a smaller image (D) Absorbs light

Answer: A. A source at the focus of a concave mirror yields a parallel beam.

Concept

The human eye works like a camera: the convex eye-lens focuses light onto the retina, forming a real, inverted image. Common defects are corrected with suitable lenses.

The ability to focus at different distances is called accommodation.

Key Points & Formulas

• Image on retina is real and inverted; the brain interprets it upright.
• Myopia (short sight): corrected with a concave lens.
• Hypermetropia (long sight): corrected with a convex lens.
• Retina contains rods (dim light) and cones (colour).

Worked Example

Q. A person cannot see distant objects clearly. Name the defect and its correction.

Difficulty with distant objects = myopia (short-sightedness).

It is corrected using a concave (diverging) lens.

Practice MCQs

Q1. The image formed on the retina is

(A) Real and inverted (B) Virtual and erect (C) Real and erect (D) Virtual and inverted

Answer: A. The convex eye-lens forms a real, inverted image on the retina.

Q2. Hypermetropia is corrected using a

(A) Convex lens (B) Concave lens (C) Plane mirror (D) Prism

Answer: A. Long-sight is corrected with a convex (converging) lens.

Concept

A magnet has two poles and produces a magnetic field. Like poles repel and unlike poles attract. The Earth itself behaves like a giant magnet, which is why a compass needle points north–south.

Magnets can be natural or artificial (electromagnets).

Key Points & Formulas

• Like poles repel; unlike poles attract.
• A freely suspended magnet aligns north–south.
• An electromagnet is made by passing current through a coil.
• Magnetic materials: iron, cobalt, nickel.

Worked Example

Q. Why does a freely suspended bar magnet always come to rest in a particular direction?

The Earth behaves as a magnet with poles near the geographic poles.

The bar magnet aligns with the Earth's magnetic field.

So it rests in the north–south direction.

Practice MCQs

Q1. Two like magnetic poles brought together will

(A) Repel (B) Attract (C) Stay neutral (D) Lose magnetism

Answer: A. Like poles repel; unlike poles attract.

Q2. A temporary magnet made using electric current is a(n)

(A) Electromagnet (B) Bar magnet (C) Lodestone (D) Compass

Answer: A. Current through a coil produces an electromagnet.

Concept

Static electricity is charge at rest; current electricity is charge in motion through a conductor. Ohm's law relates voltage, current and resistance. Cells provide the energy that drives the current.

Current has heating, lighting and magnetic effects.

Key Points & Formulas

• Ohm's law: V = IR (volt = ampere × ohm).
• Series: same current, resistances add (R = R₁+R₂).
• Parallel: same voltage, 1/R = 1/R₁ + 1/R₂.
• Primary cells (dry cell) are non-rechargeable; secondary cells (lead-acid) are rechargeable.

Worked Example

Q. A 12 V battery drives a current through a 4 Ω resistor. Find the current.

Ohm's law: I = V/R.

= 12 / 4 = 3 A.

Practice MCQs

Q1. According to Ohm's law, if voltage doubles at constant resistance, the current

(A) Doubles (B) Halves (C) Stays same (D) Becomes zero

Answer: A. I = V/R, so doubling V doubles I.

Q2. A rechargeable cell is an example of a

(A) Secondary cell (B) Primary cell (C) Dry cell (D) Solar cell

Answer: A. Secondary cells (e.g., lead-acid) can be recharged; primary cells cannot.

Concept

The NDA syllabus lists everyday devices whose basic working principle may be asked: simple pendulum, pulleys, siphon, levers, barometer, pressure cooker, thermos flask, periscope, telescope and others.

Each relies on one core physics idea — identify it.

Key Points & Formulas

• Pressure cooker: raises boiling point by increasing pressure, cooking faster.
• Thermos flask: a vacuum and silvered walls minimise conduction, convection and radiation.
• Periscope: uses two plane mirrors (or prisms) at 45°.
• Siphon: atmospheric pressure and gravity move liquid over a barrier.

Worked Example

Q. Why does food cook faster in a pressure cooker?

Trapped steam raises the pressure inside.

Higher pressure raises the boiling point of water above 100 °C.

The hotter water/steam cooks food faster.

Practice MCQs

Q1. A thermos flask keeps liquids hot mainly by reducing

(A) Heat loss by all three modes (B) Only conduction (C) Only radiation (D) Air pressure

Answer: A. Vacuum walls and silvering cut conduction, convection and radiation.

Q2. A periscope works on the principle of

(A) Reflection of light (B) Refraction of sound (C) Dispersion (D) Gravitation

Answer: A. It uses plane mirrors to reflect light along the tube.