Which of these is a derived quantity in physics, not a fundamental one?

Velocity. Velocity is derived from length and time (distance divided by time), unlike the fundamental quantities length, time, and mass. Read more: Units, Measurement and Dimensions.

Physics Fundamentals for Competitive Exams

Basic physics concepts appear regularly in competitive exams, testing everyday understanding rather than advanced theory. This article covers the core fundamentals.

MCQ Questions

General Science

Physics Fundamentals

The electromagnetic spectrum, plus six core concepts

Radio

Micro

Visible

X-ray

Gamma

Mechanics

Newton’s Three Laws

Inertia, F = ma, action-reaction

Thermal

Heat vs. Temperature

Heat flows hot to cold

Waves

Sound

Needs a medium, no vacuum travel

Electricity

Current & Ohm’s Law

V = IR

Optics

Light

Both a wave and a particle

Electromagnetism

Magnetism

Moving charge creates a field

📑 Contents

1Newton’s Three Laws of Motion
2Heat, Temperature, and Sound
3Electric Current and Ohm’s Law
4Light and the Electromagnetic Spectrum
5Test Yourself
6Electromagnetism
7Exam Point of View
8Related Topics

✊ Must Know

Newton’s Three Laws of Motion

First LawAn object at rest stays at rest, and an object in motion keeps moving at constant velocity, unless a force acts on it. This is called inertia.
Second LawForce equals mass times acceleration (F = ma). A bigger force, or a smaller mass, produces a bigger acceleration.
Third LawEvery action has an equal and opposite reaction. When you push on a wall, the wall pushes back on you with equal force.
InsteadAction and reaction forces act simultaneously, on two different objects. Neither one causes the other — there is no cause-effect relationship between them, only a paired, simultaneous interaction.
The StoryIsaac Newton published all three laws together in 1687, in his book Principia Mathematica. It became one of the most influential science books ever written.
In PracticeA car seatbelt exists purely because of the first law. Your body keeps moving forward in a crash, even after the car itself has stopped, unless something restrains it.
Galileo’s GroundworkGalileo Galilei first challenged the ancient belief that rest is an object’s natural state, laying the groundwork for Newton’s first law of inertia.
MomentumMomentum is the product of an object’s mass and velocity, measured in kg m/s. In any collision, the total momentum of a closed system stays constant, a principle called conservation of momentum, which explains why a gun recoils when fired.
Worked ExamplePush a 2 kg object with 10 newtons of force. Using F = ma, acceleration equals force divided by mass: 10 ÷ 2 = 5 m/s².
Force Is a Push or PullA force can change an object’s speed, direction, or shape. Every force needs an interaction between at least two objects.
Contact vs Non-Contact ForcesContact forces, like muscular force and friction, need the objects to physically touch. Non-contact forces, like gravitational, magnetic, and electrostatic force, act across a distance with no contact at all.

Heat, Temperature, and Sound

HeatHeat is a form of energy that flows from hotter objects to cooler ones. It depends on both temperature and how much matter is present.
TemperatureTemperature measures how hot or cold something is. It does not by itself tell you the total heat energy an object holds.
SoundSound travels as a wave through a medium, like air, water, or solids. It needs particles to carry its vibration.
InsteadSound cannot travel through a vacuum, since a vacuum has no particles to vibrate. This is why explosions in space would be silent, unlike in movies.
The TrapA hot cup of tea has a higher temperature than a full bathtub of lukewarm water. But the bathtub holds far more total heat energy, since it contains so much more water.
Why It MattersThis heat-versus-temperature gap explains why a small spark, at thousands of degrees, doesn’t set a whole room on fire. It carries very little total heat, despite its extreme temperature.

📘 Good to Know

Electric Current and Ohm’s Law

CurrentElectric current is the flow of electric charge through a conductor, like a wire.
Conductors and InsulatorsA conductor, like a metal, allows current to pass through it. An insulator, like rubber, plastic, or wood, blocks current entirely.
Complete CircuitCurrent only flows once a circuit forms an unbroken path between a cell’s two terminals. A single gap, like an open switch, stops the flow completely.
Conventional Current DirectionBy convention, current is said to flow from a cell’s positive terminal to its negative terminal through the external circuit, even though electrons physically move the other way.
Heating EffectCurrent passing through a wire generates heat, more strongly in materials with higher resistance, like the nichrome element in a heater or a glowing bulb filament.
Electric FuseA fuse uses this heating effect deliberately: a thin wire that melts and breaks the circuit once current exceeds a safe limit, protecting the circuit from overheating and fire.
Liquids and ConductivityDistilled water is a poor conductor, but dissolving an acid, base, or salt in it makes the solution conduct current well.
ElectroplatingElectroplating deposits a thin layer of one metal onto another using electric current. It coats jewellery with gold or silver, cans with tin to protect food from iron, and iron parts with zinc or chromium to resist rust and corrosion.
Static ChargesRubbing two objects together transfers charge between them. Like charges repel each other, while unlike charges attract, and this is the basis of static electricity.
Earthing and Lightning ConductorsEarthing safely transfers extra charge from an object into the ground. A lightning conductor uses this principle, protecting tall buildings by giving lightning a safe path to earth.
Benjamin FranklinIn 1752, Benjamin Franklin showed that lightning is simply a large-scale electric spark, the same phenomenon as the crackle from a charged object.
Ohm’s LawOhm’s Law describes how voltage, current, and resistance relate: V = IR. Voltage equals current times resistance.
WhyThis relationship lets engineers predict how much current will flow through a circuit, once they know its voltage and resistance.
The StoryGeorg Ohm published this law in 1827, based on careful measurements of current through wires of different lengths. German critics dismissed it at first, calling it unworthy science. The Royal Society in London recognised its importance only in 1841, awarding Ohm its Copley Medal.
Worked ExampleA circuit with 12 volts and 4 ohms of resistance carries a current of 12 ÷ 4 = 3 amperes, by direct substitution into V = IR.

🌟 Great to Know

Light and the Electromagnetic Spectrum

Wave-Particle DualityLight behaves as both a wave and a particle. Its everyday behaviours, like reflection, refraction, and dispersion into colours, follow well-established wave principles.
The SpectrumThe electromagnetic spectrum spans radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. These are ordered by wavelength and energy, from longest wavelength to shortest.
Visible LightOnly one narrow band of this entire spectrum, visible light, can actually be detected by the human eye.
The StoryWilliam Herschel discovered infrared light by accident in 1800. He split sunlight with a prism and placed thermometers along the resulting colours, as a temperature check. The thermometer he placed just past red, in what looked like empty space, showed the highest temperature of all — revealing invisible radiation beyond visible light.
In PracticeThis same infrared radiation is why night-vision cameras and remote controls work: both detect or emit light just beyond what our eyes can see.

Test Yourself

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Electromagnetism

Magnetic MaterialsIron, nickel, and cobalt are magnetic, meaning a magnet attracts them. Most other materials, like wood, plastic, and rubber, are non-magnetic.
MagnetiteMagnetite is a naturally occurring rock that behaves as a magnet, and is also a rich ore of iron.
Poles and Their RuleEvery magnet has a north and a south pole. Like poles repel each other, while unlike poles attract.
Freely Suspended MagnetsA freely suspended bar magnet always settles pointing north-south. A compass uses this exact behaviour to show direction.
The LinkMagnetism and electricity are closely linked. A moving electric charge creates a magnetic field, and a changing magnetic field can generate an electric current.
In PracticeThis deep connection, formalised as electromagnetism, isn’t just a theoretical curiosity. It is the working principle behind everyday technology like electric motors and generators, which convert directly between electrical and mechanical energy using this exact relationship.
The StoryHans Christian Oersted found this link almost by chance, in April 1820. While setting up a lecture demonstration, he noticed a compass needle near his wire twitch whenever he switched the current on. He spent the next three months testing the effect before announcing it — the first real proof that electricity and magnetism were the same underlying force.

📝 Exam Point of View

Exam Point of View

CAPF ACS 2018, Q59Sound cannot travel through a vacuum, since it needs a medium; light can, since it’s an electromagnetic wave. A question phrased as two statements tests exactly this contrast. View this question on the full CAPF ACS 2018 paper →
CDS-II 2018, Q18Of the entire electromagnetic spectrum, only visible light (option “light”) is detectable by the human eye — not infrared, radio waves, or microwaves. View this question on the full CDS-II 2018 paper →
CAPF ACS 2020, Q6A classic trap: the question asks which statement about Newton’s third law is NOT correct. The trick is that action and reaction have no cause-effect relationship — they’re simultaneous. So the statement claiming there IS a cause-effect relationship is the false one. View this question on the full CAPF ACS 2020 paper →
NDA-NA-II 2016, Q81A direct F = ma calculation: 1 newton acting on 1 kg produces exactly 1 m/s² of acceleration, by definition of the newton itself. View this question on the full NDA-NA-II 2016 paper →
The TrapDon’t confuse speed with acceleration when a question gives you a force and a mass. F = ma always solves directly for acceleration, not speed — you’d need additional information (like time) to get a speed value.

GenSci0010 — Units, Measurement and Scientific Instruments — covers the SI units (like the newton) that this article’s force and motion facts depend on.
GenSci0009 — Everyday Chemistry — the chemistry counterpart to this article’s physics fundamentals, same exam-prep format.
CAPF ACS 2018 GA Paper — contains the genuine sound-vs-light-in-vacuum PYQ cited above.
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