Title 01_VII_PHY.E-TEC._VOL-2_WS,KEY & ADD.(WPE)_1-37.pdf ✅

[e-Techno Text Book] VII - Physics (Vol-II) 1 MEMO GRAPH WORK, POWER AND ENERGY Work Power Positive work Negative work Zero work Mechanical energy Light energy Heat energy Electric energy Magnetic energy Sound energy
[e-Techno Text Book] 2 VII - Physics (Vol-II) WORK, POWER AND ENERGY WORK, POWER AND ENERGY SYNOPSIS - 1 Definition: Work is said to be done when a force produces motion. Examples: A man climbing the stairs of a house is also doing work in moving himself against the force of gravity. Mathematical Expression for work : work = Force × Distance moved in the direction of force The work done by a force on a body depends on two factors. i) Magnitude of the force. ii) Distance through which the body moves (in the direction of force) Units of work: The unit of work in C.G.S. system is ‘erg’. W = F . S = 1dyne × 1cm = 1erg 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 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. 2 m s . m 1 J = 1 kg m2 s–2 Relation between Joule and erg : 1 J = 1 N × 1 m = 105 dyn × 100 cm = 107 dyn cm or 1 J = 107 erg. Work done can be positive, negative or zero depending upon the directrion of force and direction of motion. (displacement) Work done by a force on a body (or an object) is said to be positive work done when the body is displaced in the direction of applied force. Types of work done: Positive work done :Work done by a force on a body (or an object) is said to be positive work done when the body is displaced in the direction of applied force. Examples : i) The body falling freely under the action of gravity has positive work done by the gravitational force. ii) The work done by the engine is positive. Negative work done :The work done by a force on a body is said to be negative work done when the body is displaced in a direction opposite to the direction of the force. Examples : i) When an object is lifted upward to a certain height, then the work done by the force of gravity (equal to the weight of the object) on the object is negative. ii) Work done by frictional force as force of friction and the displacement are opposite to each other. iii) Work done by a person with a suitcase on his head moving upwards in the vertical direction. Here the displacement is in the upward direction and the force of gravity acts in the downward direction. iv) When brakes are applied on a moving vehicle, work done by the braking force is negative.
[e-Techno Text Book] VII - Physics (Vol-II) 3 Zero work : If S = 0 i.e., the body does not move from its position on the application of force, then W = 0. Thus, no work is done by the force if it fails to displace the body. Ex : When a person pushes a wall but fails to move the wall, then work done by the force on the wall is zero. b) When a body moves in a direction perpendicular to the direction of the force no work is done by the force. Examples : i) When a person carrying a suitcase in his hand or on his head is walking horizontally, the work done against gravity is zero. ii) No work is done on a body when it moves along a circular path. iii) Work done by the flying aeroplane is zero as the force and displacement are perpendicular to each other. Work done against gravity : If a body is lifted vertically upwards, then the force required to lift the body is equal to it weight. So, whenever work is done against gravity, the amount of work done is equal to the product of weight of the body and the vertical distance through which the body is lifted. h F F mg ground Work done by against gravity Suppose a body of mass ‘m’ is lifted vertically upwards through a distance ‘h’. In this case the force required to lift the body will be equal to weight of the body (m × g). Work done in lifting a body = weight of body × vertical distance W = mg × h Where, m = mass of body g = acceleration due to gravity at that place h = height through which the body is lifted ACTIVITY :- Lift an object up. Work is done by the force exerted by you on the obejct. The obejct moves upwards. The force you exerted is in the direction of displacement. However, there is the force of gravity acting on the object. Which one of these forces is doing positive work ? Which one is doing negative work ?
[e-Techno Text Book] 4 VII - Physics (Vol-II) Introduction to Energy It is often said that a person A is more energetic than a person B. The meaning of this statement is that a person A can do more work than the person B. Similarly, a person after doing a lot of work gets tired and after that he is not able to do much work. It is clear that a person doing work expends something. This ‘something’ is known as the energy of the person. The energy spent by a person is equal to the work done by him. Human beings and animals get energy by eating food. It may be noted that anything which is capable of doing work has energy. For example, the steam pushes up the lid placed on the boiling water container. It means, the steam has the ability or capacity to do work. The work done by the steam on the lid is equal to the energy of the steam. Definition of energy If a person can do a lot of work we say that he has a lot of energy or he is very energetic. In physics also, anything which is able to do work is said to possess energy. Thus, energy is the ability to do work or the capacity to do work. Units of energy Unit of energy is same as that of the unit of work as work is a form of energy. So, S.I. unit of energy is Joule (J). When we say that energy of a body is 1 joule, it means, this body has the capacity to do 1 J work. Commercial unit of Energy The commercial unit (or trade unit) of energy is kilowatt-hour which is written in short form as kWh. Kilowatt-hour is usually used as a commercial unit of electrical energy. One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour. Since a kilowatt means 1000 watts, so we can also say that one kilowatt-hour is the amount of electrical energy consumed when an electrical appliance of 1000 watts is used for 1 hour. 1 kilowatt-hour is the amount of energy consumed at the rate of 1 kilowatt for 1 hour. That is, 1 kilowatt-hour = 1 kilowatt for 1 hour or 1 kilowatt-hour =1000 watts for 1 hour Note:Watt or kilowatt is the unit of electrical power but kilowatt-hour is the unit of electrical energy. Energy is a scalar quantity. Dimensional formula of energy = ML T2 2 Kinds of Energy In actual practice there are many kinds of energy, such as mechanical energy; heat energy; light energy; sound energy, electrical energy; nuclear energy; chemical energy, etc. Let us discuss about mechanical energy. Mechanical energy (M.E) The sum of kinetic energy (K.E) and potential energy (P.E) of a body is known as mechanical energy. M.E = K.E + P.E Kinetic energy:- Energy possessed by a body by virtue of it’s motion is called kinetic energy