Physics in Everyday Life

Core concepts anchored to NCERT

Motion (NCERT Class IX, "Motion" and "Force and Laws of Motion")

• Distance is the total path length (a scalar). Displacement is the shortest straight-line distance from start to finish with direction (a vector).
• Speed is distance per unit time, SI unit metre per second (m/s); a scalar. Velocity is speed in a stated direction; a vector.
• Acceleration is the rate of change of velocity, SI unit metre per second squared (m/s squared). Negative acceleration is retardation or deceleration.
• Uniform motion covers equal distances in equal time intervals.
• Three equations of motion for uniform acceleration: v = u + at; s = ut + (1/2)at squared; v squared = u squared + 2as, where u is initial velocity, v final velocity, a acceleration, t time, s distance.

Newton's laws of motion (Sir Isaac Newton, Principia, 1687)

• First law (law of inertia): a body stays at rest or in uniform straight-line motion unless acted on by an external unbalanced force. Inertia is the tendency to resist a change in state of motion and is measured by mass. This explains why passengers lurch forward when a bus brakes and why dust flies off when a carpet is beaten.
• Second law: force equals the rate of change of momentum; for constant mass, F = ma. The SI unit of force is the newton (N): one newton accelerates a one-kilogram mass at one m/s squared.
• Third law: to every action there is an equal and opposite reaction; the two forces act on different bodies. This explains rocket propulsion, the recoil of a gun, and walking (you push the ground back, it pushes you forward).

Momentum, friction and circular motion

• Momentum p = mv, SI unit kg m/s. The law of conservation of momentum states that in the absence of an external force the total momentum of a system is constant; it governs collisions and recoil.
• Friction opposes relative motion between surfaces in contact. It is useful (walking, braking, writing) and wasteful (heat loss, wear). Ball bearings and lubricants reduce it.
• Centripetal force acts towards the centre to keep a body in circular motion; the outward feeling is the apparent centrifugal effect.

Gravitation (NCERT Class IX, "Gravitation")

• Newton's universal law of gravitation: every mass attracts every other mass with a force proportional to the product of the masses and inversely proportional to the square of the distance between them.
• Acceleration due to gravity (g) at the Earth's surface is about 9.8 m/s squared. It is maximum at the poles, minimum at the equator, decreases with altitude and with depth, and is zero at the Earth's centre.
• Mass is the amount of matter in a body, constant everywhere, SI unit kilogram (kg). Weight is the force of gravity on the mass, W = mg, SI unit newton (N), and varies with g. On the Moon, weight is about one-sixth of the Earth value because the Moon's g is about 1.6 m/s squared.
• Archimedes' principle: a body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. It explains why we feel lighter in water and why ships and balloons float.

Pressure and density

• Pressure is force per unit area, SI unit pascal (Pa); one pascal is one newton per square metre. A sharp knife and a needle work by concentrating force on a small area to raise pressure.
• Density is mass per unit volume, SI unit kg per cubic metre. Relative density (specific gravity) is density compared with water and has no unit.
• Pascal's law: pressure applied to an enclosed fluid is transmitted equally in all directions; it is the basis of the hydraulic brake and the hydraulic lift.
• Atmospheric pressure at sea level is about 101.3 kilopascal (one atmosphere). It is measured by a barometer.

Work, energy and power (NCERT Class IX, "Work and Energy")

• Work is force times displacement in the direction of force, W = F times d, SI unit joule (J). No work is done if there is no displacement or if the force is perpendicular to motion.
• Energy is the capacity to do work, SI unit joule. Kinetic energy is the energy of motion, KE = (1/2)mv squared. Potential energy is stored energy of position, PE = mgh.
• Law of conservation of energy: energy can neither be created nor destroyed, only transformed; the total energy of an isolated system is constant.
• Power is the rate of doing work, P = W per t, SI unit watt (W); one watt is one joule per second. One horsepower is about 746 watts. The commercial unit of electrical energy is the kilowatt-hour (one unit equals one kWh).

Light and optics (NCERT Class X, "Light: Reflection and Refraction", "The Human Eye")

• Light travels in straight lines (rectilinear propagation) at about 3 × 10 to the power 8 metres per second in vacuum.
• Reflection: the angle of incidence equals the angle of reflection. A plane mirror gives a virtual, erect, laterally inverted image of the same size, formed as far behind the mirror as the object is in front.
• Spherical mirrors: a concave mirror (converging) can magnify and is used as a shaving or make-up mirror, in vehicle headlights and in solar cookers. A convex mirror (diverging) always gives a small, erect, virtual image and a wide field of view, so it is used as a vehicle rear-view mirror and at blind road corners.
• Refraction: light bends when it passes between media of different optical density. It explains the bent straw, the apparently shallow pool, the twinkling of stars and the early or late visibility of the Sun. The refractive index measures how much a medium bends light.
• Lenses: a convex (converging) lens corrects hypermetropia (long sight) and is used as a magnifying glass; a concave (diverging) lens corrects myopia (short sight). The power of a lens is measured in dioptres (reciprocal of focal length in metres).
• Dispersion: a glass prism splits white light into the seven-colour spectrum VIBGYOR (violet, indigo, blue, green, yellow, orange, red). Violet bends most, red least. A rainbow is dispersion plus internal reflection in raindrops.
• Total internal reflection occurs when light in a denser medium strikes the boundary beyond the critical angle; it makes a mirage and powers optical fibre communication.

Sound (NCERT Class IX, "Sound")

• Sound is a longitudinal mechanical wave that needs a material medium, so it cannot travel through vacuum.
• Its speed in air at room temperature is about 343 m/s (often taken as 340 m/s), far slower than light, which is why lightning is seen before thunder is heard. Sound travels faster in liquids and fastest in solids.
• The human audible range is about 20 Hz to 20,000 Hz. Below 20 Hz is infrasound; above 20,000 Hz is ultrasound, used in SONAR (echo ranging), medical scans and cleaning.
• Frequency (pitch) is measured in hertz (Hz); amplitude governs loudness, measured in decibels (dB). An echo is reflected sound and needs a minimum distance of about 17 metres for the reflection to be heard separately.

Heat and thermodynamics (NCERT Class VII "Heat", Class X integration)

• Heat is energy in transit due to a temperature difference; SI unit joule. Temperature measures the degree of hotness; SI unit kelvin (K).
• Heat transfers by conduction (through solids, particle to particle), convection (in fluids, by bulk movement) and radiation (no medium needed, as the Sun's heat reaching Earth).
• Temperature scales: Celsius, Fahrenheit and Kelvin. 0° Celsius equals 273.15 K; water freezes at 0 and boils at 100° Celsius at sea level. Conversion: F = (9/5)C + 32.
• Latent heat is the heat absorbed or released during a change of state at constant temperature; it explains why sweating and an earthen pot cool by evaporation.
• A bimetallic strip, a mercury or alcohol thermometer, and the gap left in railway tracks all rely on thermal expansion. Most substances expand on heating; water is anomalous and expands on freezing, so ice floats.

Electricity (NCERT Class X, "Electricity")

• Electric current is the rate of flow of charge, SI unit ampere (A). Conventional current flows from positive to negative; electrons flow the opposite way.
• Potential difference (voltage) is measured in volts (V); resistance in ohms (Greek omega).
• Ohm's law: V = I times R, valid at constant temperature (Georg Ohm).
• Electric power P = V times I = I squared times R, SI unit watt.
• Conductors in series share one path (current same, voltage divides); in parallel each branch is independent (voltage same, current divides). Household wiring uses parallel connection so each appliance works on its own.
• A fuse or miniature circuit breaker (MCB) protects against excess current; earthing of metal appliances prevents shock. India's domestic supply is alternating current at about 230 V, 50 hertz.

Magnetism and electromagnetism (NCERT Class X, "Magnetic Effects of Electric Current")

• Like magnetic poles repel, unlike poles attract. A freely suspended magnet points north to south.
• Oersted (1820) found that a current-carrying wire produces a magnetic field around it, the basis of the electromagnet and the electric motor (electrical to mechanical energy).
• Faraday's electromagnetic induction (Michael Faraday, 1831): a changing magnetic field induces a current. This is the basis of the electric generator (mechanical to electrical energy) and the transformer (which steps voltage up or down using AC).