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2 WORK, ENERGY AND POWER Topic-1 Work, Energy and its Conservation Concepts covered: Work, Energy (Potential and Kinetic) Conservation of Energy Revision Notes Energy is the capacity of a body to do work. Its S.I. unit is joule. When a non-zero force (F) is applied on an object, it displaces by (d) by making an angle (q) with the direction of force. Then work done mathematically, is defined as. W = Fd cos q Work is said to be done only when force applied on a body makes the body move. S.I. unit of work is joule. Work is equal to force multiplied by displacement in the direction of force. Work can be positive, negative or zero. If the angle between force and the displacement is acute, then work is said to be positive. If the angle between force and the displacement is 90 i.e., displacement is perpendicular to the force applied, work is said to be zero. If the angle between force and the displacement is obtuse, the work is said to be negative. When a coolie walks horizontally while carrying a load on his head, no work is done against the force of gravity. When a body rotates in a circular path, no work is done against the centripetal force, as force and displacement are normal to each other. Work done can be zero if : (i) force applied is zero i.e., no force acts on the body. (ii) displacement of body is zero. (iii) angle between force and displacement is 90 . C.G.S. unit of energy or work is erg. One joule of work is said to be done when a force of 1 Newton displaces a body through a distance of 1 metre in its own direction. 1 erg of work is said to be done when a force of 1 dyne displaces a body through a distance 1 cm in its own direction. One kilowatt hour (1 kWh) is the energy spent (or work done ) by a source of power 1 kW in 1 hour. Mechanical energy is of two types i.e., 1 2 (i) Kinetic energy = mv 2 (ii) Potential energy = mgh Kinetic energy is the energy possessed by a body by virtue of its motion. Examples include a moving train, a running boy, etc. Types of kinetic energy : (i) Translation kinetic energy (e.g., a car moving in straight path, a freely falling body posses translational kinetic energy) (ii) Rotational kinetic energy (e.g., a spinning top, a rotating fan posses rotational kinetic energy) (iii) Vibrational kinetic energy (e.g., a wire clamped at both the ends when struck in the middle vibrates, possessing vibrational kinetic energy) Potential energy is the energy possessed by a body by virtue of its position or configuration. Potential energy is of two types : (i) Elastic potential energy (ii) Gravitational potential energy
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