Physics in Everyday Life: Notes for UPSC, SSC & Railway Exams

Units and measurement

The International System of Units (SI) has seven base units. Every other unit is built from them. Exams love to ask which unit is base and which is derived, and what a named unit measures.

Motion and force

• First law (inertia): a body stays at rest or in uniform motion unless an external force acts. Inertia of rest explains why dust leaves a carpet when it is beaten; inertia of motion explains why an athlete keeps running past the finish line.
• Second law: force = rate of change of momentum, so F = ma. Momentum p = mv. A cricketer pulls the hands back while catching to increase the time and reduce the force.
• Third law: every action has an equal and opposite reaction, acting on different bodies. Rockets, recoil of a gun and swimming all use it.
• Acceleration due to gravity near Earth is about 9.8 m/s². On the Moon it is about one-sixth of this.

Pressure and fluids

• Pressure = force / area. Sharp knives and narrow nails work because a small area means high pressure; wide straps on bags spread the load.
• Pascal's law: pressure applied to an enclosed liquid is transmitted equally in all directions. Hydraulic lifts and hydraulic brakes use it.
• Archimedes' principle: a body in a fluid feels an upward buoyant force equal to the weight of fluid displaced. A ship floats because it displaces a large volume of water.
• Ice floats because it is less dense than liquid water. Water is densest at about 4 °C, which is why lakes freeze from the top and fish survive below.
• Atmospheric pressure at sea level is about 101 kPa (76 cm of mercury), measured with a barometer.

Heat and temperature

Conversions: K = °C + 273.15 and F = (9/5)C + 32. Absolute zero is 0 K, about −273 °C. Normal human body temperature is about 37 °C (98.6 °F).

• Boiling point depends on pressure. On mountains the air pressure is lower, water boils below 100 °C and food takes longer to cook. In a sealed cooker the pressure is higher and water boils above 100 °C.
• Heat travels by conduction (solids, through contact), convection (fluids, by moving currents — sea breeze, boiling water) and radiation (no medium needed — heat from the Sun).
• A vacuum flask cuts conduction and convection with a vacuum gap and reduces radiation with silvered walls.
• Dark, rough surfaces absorb and emit heat well; white, shiny surfaces reflect it — hence light-coloured summer clothes.

Sound

• Sound is a longitudinal mechanical wave; light is an electromagnetic wave and needs no medium.
• Audible range: about 20 Hz to 20,000 Hz. Ultrasound (above 20 kHz) is used in SONAR, medical scans and by bats; infrasound (below 20 Hz) is produced by earthquakes and heard by elephants and whales.
• Loudness is measured in decibels (dB); pitch depends on frequency; quality (timbre) on waveform.
• An echo is heard only if the reflecting surface is at least about 17 m away, because the ear keeps a sound for about 0.1 s.
• Sound travels faster in warm air than in cold air, and faster in humid air than in dry air.

Light

• Speed of light in vacuum is about 3 × 10^8 m/s; sunlight takes about 8 minutes 20 seconds to reach Earth.
• Refraction is the bending of light when it changes medium: a pool looks shallower and a pencil in water looks bent.
• Total internal reflection happens when light goes from a denser to a rarer medium at more than the critical angle. It explains the sparkle of diamonds, mirages and optical fibres.
• Dispersion: a prism splits white light into VIBGYOR. Violet bends the most, red the least. A rainbow forms by refraction, dispersion and internal reflection in raindrops.
• Scattering: tiny air molecules scatter short wavelengths (blue) much more than long ones. At sunrise and sunset light travels a longer path, blue is scattered away and the Sun looks reddish. Red is scattered least, so it carries farthest in fog — hence red danger lights.

Electricity at home

• Ohm's law: V = IR (at constant temperature). Resistance is in ohms.
• Power P = VI = I²R = V²/R. A 1,000 W iron draws about 4.5 A at 220 V.
• Energy bills count units: 1 unit = 1 kilowatt-hour = 3.6 × 10^6 J. A 100 W bulb used for 10 hours consumes 1 unit.
• Indian homes get alternating current at about 220–230 V and 50 Hz.
• Household appliances are connected in parallel so each gets full voltage and can be switched independently.
• Safety: a fuse (low-melting wire, in series with the live wire) or an MCB cuts the circuit on overload; the earth wire carries leakage current safely to the ground from metal-bodied appliances.