NCERT Science: Work, Energy, Power and Sound

The one-line takeaway

Work is done when a force moves a body through a distance; energy is the capacity to do work and is conserved (only transformed, never created or destroyed); power is the rate of doing work. Sound is a longitudinal mechanical wave that needs a medium, and its properties explain echoes, ultrasound, SONAR and noise.

Work

• Work is done when a force acts on a body and the body moves in the direction of the force. Work equals force times displacement in the direction of the force. SI unit the joule (one joule is one newton acting through one metre).
• No work is done if there is no displacement (pushing a wall that does not move), or if the displacement is perpendicular to the force (carrying a load horizontally).
• Work can be positive (force and motion in the same direction), negative (opposite, as friction does) or zero.

Energy

• Energy is the capacity to do work; its SI unit is also the joule. Forms include mechanical, heat, light, sound, chemical, electrical and nuclear energy.
• Kinetic energy is the energy of a moving body, equal to half times mass times velocity squared. It rises sharply with speed (doubling speed quadruples kinetic energy), which is why high-speed impacts are so destructive.
• Potential energy is stored energy of position or configuration; gravitational potential energy equals mass times g times height. A stretched bow and a raised hammer store potential energy.
• Law of conservation of energy: energy can be transformed from one form to another but the total energy is constant; it can neither be created nor destroyed. A falling body converts potential energy into kinetic energy; a pendulum swaps the two continuously.

Power

• Power is the rate of doing work or transferring energy; SI unit the watt (one watt is one joule per second). One kilowatt is 1,000 watt; one horsepower is about 746 watt.
• The kilowatt-hour is the commercial unit of electrical energy (the "unit" on the electricity bill); one kilowatt-hour equals 3.6 million joule (3.6 megajoule).

Sound: a mechanical wave

• Sound is produced by a vibrating body and travels as a longitudinal mechanical wave (the particles of the medium vibrate back and forth along the direction the wave travels, forming compressions and rarefactions).
• Sound needs a material medium (solid, liquid or gas) and cannot travel through a vacuum; this is why there is no sound in outer space.
• Speed of sound is about 343 metre per second in air at room temperature, faster in water and fastest in solids (the particles are closer and pass on the vibration quickly). It increases with temperature.

Properties of sound

• Frequency (hertz) is the number of vibrations per second and determines pitch (high frequency, high pitch). Amplitude determines loudness. Wavelength is the distance between two successive compressions.
• The audible range for humans is about 20 hertz to 20,000 hertz. Below 20 hertz is infrasound (produced by large animals, earthquakes); above 20,000 hertz is ultrasound (bats and dolphins use it).
• Echo is the repetition of sound by reflection; the human ear distinguishes an echo only if the reflected sound returns after at least about 0.1 second. Reverberation is the persistence of sound by repeated reflection in a hall.

Applications of sound

• Ultrasound: medical imaging (sonography), breaking kidney stones, cleaning machine parts and detecting flaws in metal castings.
• SONAR (sound navigation and ranging): uses ultrasound echoes to measure the depth of the sea and to detect submarines, shoals of fish and submerged objects, central to naval and maritime security.
• Noise pollution: unwanted, loud sound that harms health; its control is a public-health and environmental concern.

Key terms to fix

• Joule: SI unit of work and energy. Watt: SI unit of power (one joule per second).
• Kinetic versus potential energy: energy of motion versus stored energy of position.
• Conservation of energy: total energy is constant, only transformed.
• Sound is longitudinal and needs a medium; speed in air about 343 metre per second.
• Audible range: 20 hertz to 20,000 hertz; below is infrasound, above is ultrasound.

CAPF angle

SONAR is a core maritime-security and submarine-detection technology, relevant to coastal-security duties. The energy-and-impact relationship (kinetic energy rising with the square of speed) underlies ballistics and the destructive power of projectiles and explosives. Ultrasound is used in field medical diagnostics and in non-destructive testing of equipment. Noise pollution and blast over-pressure are occupational-health considerations for forces using firearms and explosives.

Authored practice

1. The commercial unit of electrical energy, the kilowatt-hour, equals how many joule? (Answer: 3.6 million joule, that is 3.6 megajoule.) Authored practice, not a verbatim PYQ.
2. SONAR works on the principle of: (a) reflection of light (b) reflection of ultrasound (echo ranging) (c) refraction of radio waves (d) conduction of heat. (Answer: b.) Authored practice, not a verbatim PYQ.