General physics facts for prelims: SI units, mechanics, electricity, sound and light

Physics is the science of matter, energy and their interaction. For prelims, physics questions rarely ask for derivations; they ask for units, instruments, definitions and worked values that recur year after year. This article collects those facts in one place: the seven base SI quantities, scalar and vector quantities, the mechanics results that appear as one-mark questions, the electricity and magnetism formulas, and the sound, light and heat facts that prelims tests.

SI units and measuring instruments

The International System of Units (SI) rests on seven base quantities, each with its own unit and symbol. Every other physical quantity is a combination of these seven, and prelims regularly asks which quantity a given unit measures.

Derived units combine base units. A newton measures force, a joule measures energy, a watt measures power, a pascal measures pressure, an ohm measures resistance, a farad measures capacitance, a henry measures inductance and a tesla measures magnetic flux density.

Asked in SSC CGL 2026

Scalar and vector quantities

A scalar quantity has magnitude only, while a vector quantity has both magnitude and direction. The distinction is a favourite one-mark question, and the test is simple: if a quantity needs a direction to be fully described, it is a vector.

Speed is scalar and velocity is its vector counterpart. Distance is scalar and displacement is its vector counterpart. Weight is a force and therefore a vector, while mass is a scalar.

Asked in RRB NTPC

Mechanics: the worked values prelims repeats

A velocity-time graph carries two facts in one picture. Its slope gives acceleration, and the area under the curve gives displacement. In a distance-time graph the slope gives speed.

In every collision, total momentum is conserved. In a perfectly elastic collision kinetic energy is also conserved, while in a perfectly inelastic collision the colliding bodies stick together and the loss of kinetic energy is maximum.

In simple harmonic motion the total mechanical energy is 1/2 mω²A², where m is the mass, ω the angular frequency and A the amplitude. It depends on the square of the amplitude, so doubling the amplitude quadruples the total energy.

A body rolling down an incline accelerates fastest as a solid sphere, next as a solid disc and slowest as a ring, because more of its mass sits farther from the axis in the ring, increasing its moment of inertia.

Free fall near the Earth has acceleration g, about 9.8 m/s². A projectile reaches its maximum height when its vertical velocity becomes zero, and its range is maximum at a launch angle of 45 degrees.

Electricity and magnetism

The energy stored in a capacitor is 1/2 CV², which equals Q²/2C. Doubling the capacitance at constant voltage doubles the stored energy, and doubling the voltage quadruples it.

The power dissipated in a resistor is V²/R, which equals I²R. One volt is one joule per coulomb: the potential difference between two points is one volt when one joule of work moves one coulomb of charge between them.

An electric motor converts electrical energy into mechanical energy, while a dynamo or generator converts mechanical energy into electrical energy. A transformer changes alternating voltage, stepping it up or down, and works only on alternating current.

A magnetic field is measured in tesla. The Earth behaves like a giant magnet, and a freely suspended magnet aligns along the magnetic north-south direction.

Sound, light and waves

Sound needs a medium. It travels fastest in solids, slower in liquids and slowest in gases, and it cannot travel through a vacuum. In air its speed is about 331 metres per second at 0 °C and rises with temperature.

Human hearing spans roughly 20 Hz to 20,000 Hz. Sound below 20 Hz is infrasonic, sound above 20,000 Hz is ultrasonic, and bats navigate with ultrasonic echoes.

Light travels in a vacuum at about 3 x 10 to the power 8 metres per second, the fastest speed in nature. It is an electromagnetic wave, so unlike sound it needs no medium.

Dry ice is solid carbon dioxide. It turns directly from solid to gas, a process called sublimation, and it is used to keep things cold without leaving any liquid behind.

The colour of an object is the colour it reflects. A red object looks red because it reflects red light and absorbs the rest, and a white object reflects all colours of visible light.

Heat, thermodynamics and modern physics

Heat flows from a hotter body to a colder body until both reach the same temperature. Water has an unusually high specific heat, which is why it is used as a coolant and why coastal places have mild climates.

Nuclear fission splits a heavy nucleus such as uranium-235 into lighter nuclei, releasing a large amount of energy. A chain reaction is self-sustaining when each fission releases neutrons that trigger further fissions.

In the Bohr model of the atom, electrons move in fixed orbits around the nucleus and occupy only certain allowed energy levels. An electron jumps between levels by absorbing or emitting energy as light.

Asked in SSC CGL 2026

Prelims practice