Title SBV VII PHYSICS.pdf ✅

Class - VII 5 Physics Work and Energy Some work is done in moving the box. If the same force is used to push the box so that displacement is 4 m, the work done will be doubled. Thus, we can conclude that more the displacement of a body, more is the work done. (a) (b) (c) Note: For doing the work, two conditions are necessary: (i) There must be force acting on the body. (ii) The body must move in the direction of applied force. Mathematically, work is defined as the product of force and displacement through which force acts. If F is the applied force, ‘S’ is the displacement in the direction of applied force and W is the work done, then: Work done = Force × Displacement through which force acts. W = F × S 5.2.1 Units of Work The unit of work in C.G.S. system is ‘erg’. W = F . S = 1dyne × 1cm = 1erg (i) erg : Work done is said to be one ‘erg’ if a force of one dyne displaces the body through a distance of 1 cm along the direction of force. 1 erg = 1 dyne × 1 cm = 1 g cm s–2 × cm or 1 erg = 1 g cm2 s–2 The unit of work in S.I. system is joule (J) W = F . S = 1N × 1m = 1 joule (ii) Joule : Work is said to be one joule, if a force 1 newton displaces a body through a distance of 1 m along the direction of force. 1 J = 1 N × 1 m = 1 kg × m× s-2 × m 1 J = 1 kg m2 s–2 5.2.2 Relation between Joule and erg : 1 J = 1 N × 1 m = 105 dyne × 100 cm = 107 dyne cm or 1 J = 107 erg. Work done can be positive, negative or zero depending upon the direction of force and direction of motion. (Displacement) 5.2.3 Types of Work (i) Positive work done : Work done by a force on a body (or an object) is said to be F S positive work done when the body is displaced in the direction of applied force.