# Physics: from force to the nucleus

> These study notes explain Physics: matter, force, motion, energy, electricity, magnetism, light, sound, heat for the Group 4 / VAO (Combined Civil Services Examination-IV) exam in plain language: the same reviewed notes the TNPSC Guide app shows, published by our editors with their source. Read the key points first, then practise the unit's questions. For exam facts such as dates and vacancies, always check tnpsc.gov.in.

Last updated: 2026-11-02 · By TNPSC Guide editorial team
URL: https://www.tnpscguide.com/exams/group-4/notes/gs-science-physics

## Why the exam asks this

Five questions on this topic came in the 2022, 2024 and 2025 papers; four are answered directly in these pages. 2022 Q160 asked the law behind the hydraulic lifts that raise vehicles in workshops (final key: Pascal's law). 2022 Q161 asked which equation is not an equation for force (final key: F = ΔP × t). 2022 Q194 matched the galvanometer, voltmeter, ammeter and rheostat with their uses (final key: 4 3 2 1). 2024 Q150 asked for the wrong statement among the conditions in which the Doppler effect does not occur (final key: "1 only"). 2025 Q173 gave as its assertion that cobalt-60 is used in cancer treatment; our book says cobalt-60 is used for skin cancer, and the reason part of that question is not in the pages we read.

## Mechanics and inertia

Mechanics is the study of the effects of force on a body. It has two branches, statics and dynamics, and dynamics again has two, kinematics and kinetics. Inertia is the property by which a body stays at rest or in uniform straight-line motion until an unbalanced external force acts; it is of three kinds: inertia of rest, of motion and of direction. Momentum is the product of a moving body's mass and velocity (momentum = mass × velocity). Example: when a town bus stops suddenly, standing passengers lean forward because the body tries to stay in motion (inertia of motion).

## Force, impulse and the third law

The SI gravitational unit of force is the kilogram force (kgf) and the CGS unit the gram force (gf): 1 kg f = 1 kg × 9.8 ms⁻² = 9.8 newton; 1 g f = 1 g × 980 cms⁻² = 980 dyne. A very large force acting for a very short time is an impulsive force. Impulse J = F × t. By Newton's second law F = Δp/t, so Δp = F × t: impulse equals the change in momentum. The unit of impulse is kg m s⁻¹ or newton second. When the time of impact grows, the impulsive force falls; that is why vehicles have springs and shock absorbers. Newton's third law: every force has an equal and opposite force, and the two act on two different bodies (F_A = − F_B). A bird flying, swimming and the recoil of a gun are examples. Example: when a two-wheeler crosses a speed breaker on a village road, the shock absorber stretches the time of impact and lowers the force on our body.

## Gravitation, mass and weight

The gravitational constant G is 6.674 × 10⁻¹¹ N m² kg⁻² (SI); the mean value of the acceleration due to gravity g at sea level is 9.8 m s⁻². The Earth's radius is about 6400 km; its mass M = 5.972 × 10²⁴ kg; g = GM/R². The books give different exact values for the radius, so remember only "about 6400 km". The radius is largest at the equator, so g is least there and greatest at the poles; g falls with height or depth and is zero at the Earth's centre. The unit of mass is the kilogram; weight W = mass (m) × g; weight is a vector, and its unit is the newton (N). In free fall R = m(g − g) = 0: this is weightlessness; an astronaut moves with the same acceleration as the spacecraft and so appears weightless. The sprouting and growth of roots under the Earth's pull is geotropism; Newton's law of gravitation helps find new stars and planets. Example: the roots of a paddy seedling planted in the field always grow downward; that is geotropism.

## Pascal's law and the hydraulic press

Pascal's law is named after Blaise Pascal (1623 – 1662), the French mathematician and physicist: an external force on an incompressible liquid is passed on evenly in all directions. The hydraulic press is based on Pascal's law. Since F₂ = F₁ × A₂/A₁, the force on the larger piston is greater, so hydraulic systems are called force multipliers. A hydraulic lift works by Pascal's law, not by Newton's laws. Example: the hydraulic lift in a workshop that raises a heavy car when a small piston is pushed; 2022 Q160 asked exactly this.

## Light: scattering and lenses

The scattering of light by the particles of a colloid is Tyndall scattering, or the Tyndall effect. When light of one colour passes through a transparent medium and the scattered ray's wavelength and frequency change, it is Raman scattering. Lines with the same frequency as the incident ray are Rayleigh lines; lines of new frequency are Raman lines. Lines of lower frequency than the incident ray are Stokes lines; those of higher frequency are anti-Stokes lines. A convex lens (double convex) brings a beam to a point, so it is a converging lens; a concave lens (double concave) spreads it, so it is a diverging lens. Example: "kuvi" means to gather and "kuzhi" means a hollow; the gathering lens is convex, the spreading lens concave.

## Heat

One unit of temperature is 1/273.16 of the triple point of water; a difference of one degree Celsius equals one kelvin. The Standard 10 book gives the conversions K = C + 273, K = (F + 460) × 5/9 and 0 K = −273°C; set sums in this form of the book. Heat always flows from a body at higher temperature to one at lower temperature. The SI unit of heat energy is the joule (J) and of temperature the kelvin (K). The ideal gas equation is PV = RT, where R is the universal gas constant (8.31 J mol⁻¹ K⁻¹). For the same rise in temperature a liquid expands more than a solid, and a gas more than a solid or a liquid. Example: when we hold a hot cup of tea, heat flows from the cup to the hand, not from the hand to the cup.

## Current electricity

The SI unit of current is the ampere (A); 1 ampere = 1 coulomb / 1 second. Uses of circuit components: the resistor fixes the amount of current, the rheostat selects it, the ammeter measures current, the voltmeter measures potential difference, and the galvanometer detects current and its direction. The German physicist Georg Simon Ohm set out the relation between current and potential difference: at constant temperature the current is directly proportional to the potential difference; V = IR; the SI unit of resistance is the ohm (Ω).

## Heating effect and resistance

Joule's law of heating H = I²Rt: the heat is directly proportional to the square of the current, to the resistance and to the time. Heating appliances have a coil of nichrome, an alloy of nickel and chromium; a fuse wire has a low melting point; the filament is tungsten. The unit of potential and potential difference is the volt (V); the unit of resistivity is the ohm metre (Ωm), and its reciprocal is conductivity (σ = 1/ρ). In series the effective resistance is more than the highest single resistance; in parallel it is less than the lowest. 1 horsepower = 746 watt. Example: the electric iron and the electric stove at home heat up through a nichrome coil.

## Sound and light

Sound of frequency 20 Hz to 20,000 Hz is audible. Below 20 Hz is infrasonic; earthquakes, ocean waves and whales are the examples. Above 20,000 Hz is ultrasonic; mosquitoes, dogs, bats and dolphins hear it. Sound waves need a medium, are longitudinal and travel at 340 m s⁻¹ (NTP). Light waves need no medium, are transverse, have a wavelength of 4 × 10⁻⁷ m to 7 × 10⁻⁷ m and travel at 3 × 10⁸ m s⁻¹. To hear an echo, the source and the reflecting surface must be at least 17.2 m apart. Solids are more elastic than liquids and gases, so sound travels fastest in solids. Example: in a thunderstorm we see the lightning first and hear the thunder later; light is far faster than sound.

## The Doppler effect

There are four conditions in which the Doppler effect does not occur: (i) the source (S) and the listener (L) are at rest; (ii) S and L move keeping equal distance, that is, the gap does not change; (iii) S and L move at right angles to each other; (iv) the source is at the centre of a circle and the listener moves along the circle. "The listener at rest and the source moving towards the listener" is not one of the four; then the Doppler effect occurs. Uses of the Doppler effect: measuring a vehicle's speed, finding the distance of a satellite, RADAR (Radio Detection And Ranging) and SONAR (Sound Navigation And Ranging). Example: two friends cycling side by side at the same speed in the same direction keep the same gap; this is the second no-Doppler condition.

## Radioactivity

Elements with atomic number above 82 (uranium, radium) give out radiation on their own. Technetium (43) and promethium (61), with atomic numbers below 82, are radioactive too. Units of radioactivity: the becquerel (Bq) is the SI unit, one decay per second; the curie is 3.7 × 10¹⁰ decays per second, about the decay of 1 gram of radium 226; the rutherford (Rd) = 10⁶ decays; the roentgen is 2.58 × 10⁻⁴ coulomb per kilogram of air. The curie is not the SI unit. Example: the market may still use old measures such as padi and marakkal, but the unit the government accepts is the kilogram; in the same way, there are the curie and the rutherford, but the SI unit is the becquerel.

## Nuclear fusion and fission

Nuclear fusion needs a very high temperature of 10⁷ to 10⁹ K and high pressure, so it is called thermonuclear fusion; it is the source of the energy of the Sun and the stars. In the Sun 620 million metric tons of hydrogen fuse every second, releasing 3.8 × 10²⁶ joule a second; on reaching the Earth this is 1.4 kilojoule per unit area per second. Fission is the splitting of a heavy nucleus into lighter ones; it happens even at room temperature and gives out alpha, beta and gamma rays. Fusion is the joining of two light nuclei; it needs high temperature and pressure and gives out alpha rays, positrons and neutrinos. Example: the energy of the summer sunshine on our fields comes from the fusion in the Sun.

## Uses of radioisotopes

In medicine: sodium-24 keeps the heart working evenly, iodine-131 treats goitre, iron-59 anaemia, phosphorus-32 skin disease, and cobalt-60 (Co⁶⁰) and gold-198 (Au¹⁹⁸) skin cancer. In farming, phosphorus P-32 raises crop yield. Californium-252 (Cf²⁵²) detects explosives in air luggage, and americium-241 (Am²⁴¹) is used in smoke detectors. Radiocarbon dating finds the age of fossils, old paintings and monuments. The same page gives the age of the Earth as about 4.54 × 10⁹ years; do not link this age to the carbon method. Example: think of the rooms of a hospital: sodium in the heart room, iodine in the neck room, iron in the blood room, phosphorus in the skin room, and phosphorus in the field as well.

## Nuclear energy in India

The Atomic Energy Commission (AEC) of India was set up in Mumbai in August 1948; its first chairman was Dr Homi Jehangir Bhabha. According to the book (2024 reprint), nuclear energy is the fifth resource in India's power generation. India's first nuclear power station is Tarapur; Tamil Nadu has nuclear power stations at two places, Kalpakkam and Kudankulam. Apsara is the first nuclear reactor built in Asia and in India; the other reactors are Cirus, Dhruva and Purnima. A power station and a reactor are different words: the first station is Tarapur, the first reactor Apsara. The coolants of a reactor are water, air and helium; a thick lead wall stops radiation from spreading. Example: the names of both Tamil Nadu nuclear power stations begin with a letter of the "ka" row: Kalpakkam (ka) and Kudankulam (koo).

**Key facts**

| | |
| --- | --- |
| Kinds of inertia | three: of rest, of motion, of direction (Std 10, p. 11) |
| Kilogram force / gram force | 1 kg f = 9.8 newton / 1 g f = 980 dyne (Std 10, p. 7) |
| Impulse | J = F × t = Δp; F = Δp/t (Std 10, p. 7) |
| G / g | 6.674 × 10⁻¹¹ N m² kg⁻² / 9.8 m s⁻² (Std 10, p. 9) |
| Earth's radius / mass | about 6400 km / 5.972 × 10²⁴ kg (Std 10, p. 9) |
| Pascal | 1623 – 1662; hydraulic press, force multiplier (Std 9, p. 31) |
| Temperature conversion | K = C + 273; K = (F + 460) × 5/9; 0 K = −273°C (Std 10, p. 34) |
| Universal gas constant R | 8.31 J mol⁻¹ K⁻¹ (Std 10, p. 39) |
| Ohm's law / Joule's law | V = IR / H = I²Rt (Std 10, pp. 46, 51) |
| Horsepower | 1 HP = 746 watt (Std 10, p. 55) |
| Audible sound | 20 Hz to 20,000 Hz (Std 10, p. 61) |
| Speed of sound / light | 340 m s⁻¹ / 3 × 10⁸ m s⁻¹ (Std 10, p. 61) |
| Echo | at least 17.2 m (Std 10, p. 69) |
| Natural radioactivity | atomic number above 82; technetium 43, promethium 61 (Std 10, p. 75) |
| Curie | 3.7 × 10¹⁰ decays a second (Std 10, p. 76) |
| Fusion temperature | 10⁷ to 10⁹ K (Std 10, p. 81) |
| Fusion in the Sun | 620 million metric tons of hydrogen a second (Std 10, p. 81) |
| Atomic Energy Commission | August 1948, Mumbai; first chairman Homi Jehangir Bhabha (Std 10, p. 84) |
| First power station / first reactor | Tarapur / Apsara (Std 10, p. 84) |

> **Exam tip:** Memory hooks
"Impulse multiplies, force divides: J = F × t, F = Δp/t." "g rises at the poles, falls at the equator, is zero at the centre." "A lift means Pascal." "The ammeter measures current, the voltmeter potential difference, the galvanometer detects direction." "Infrasonic below, ultrasonic above; twenty to twenty thousand in between." "SI means becquerel." "Station Tarapur, reactor Apsara."

**Easy to confuse**

| | |
| --- | --- |
| F = Δp/t / F = Δp × t | the first is an equation for force / the second is wrong; Δp = F × t is impulse |
| G / g | the fixed gravitational constant 6.674 × 10⁻¹¹ / the acceleration due to gravity, which changes from place to place |
| Mass / weight | unit kilogram / W = mg, a vector, unit newton |
| Ammeter / galvanometer | measures current / detects current and its direction |
| Resistor / rheostat | fixes the amount of current / selects it |
| Infrasonic / ultrasonic | below 20 Hz, whales, earthquakes / above 20,000 Hz, dogs, bats |
| Rayleigh lines / Raman lines | the same frequency as the incident ray / a new frequency |
| Stokes / anti-Stokes | lower frequency than the incident ray / higher frequency |
| Fission / fusion | a heavy nucleus splits, even at room temperature / light nuclei join, at high temperature |
| Becquerel / curie | SI unit, one decay a second / 3.7 × 10¹⁰ decays; the curie is not the SI unit |
| Carbon dating / age of the Earth | fossils, paintings, monuments / 4.54 × 10⁹ years; do not link the two |
| Cobalt-60 / iodine-131 | skin cancer / goitre |

> **Exam tip:** Quick check
1. By which law does a workshop's hydraulic lift work? Answer: Pascal's law.
2. Which instrument detects current and its direction? Answer: the galvanometer.
3. What is the SI unit of radioactivity? Answer: the becquerel (Bq).
4. Which was the first nuclear reactor built in Asia and India? Answer: Apsara.
5. What is the least distance needed to hear an echo? Answer: 17.2 m.

Source: அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Science, Standard X, Tamil medium; Samacheer Kalvi) (10ஆம் வகுப்பு அறிவியல், அலகு 1 (இயக்க விதிகள்) பக்கம் 7, 9, 11; அலகு 2 (ஒளியியல்) பக்கம் 19; அலகு 3 (வெப்ப இயற்பியல்) பக்கம் 34, 39; அலகு 4 (மின்னோட்டவியல்) பக்கம் 44, 46, 51, 55; அலகு 5 (ஒலியியல்) பக்கம் 61, 69; அலகு 6 (அணுக்கரு இயற்பியல்) பக்கம் 75-76, 81-82, 84; 9ஆம் வகுப்பு அறிவியல், அலகு 3 (பாய்மங்கள்) பக்கம் 31)

## Sources

- [Official syllabus page](https://www.tnpsc.gov.in/english/syllabus.html)
- [Notifications](https://www.tnpsc.gov.in/English/Notification.aspx)