Work, Energy and Power - Unit 4 - Science (Class 9)
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The document explains the concepts of work, energy, and power, including definitions, units, types of energy (mechanical, kinetic, potential), and the principle of conservation of energy. It outlines the formulae for calculating work (W = F x d), kinetic energy (KE = 1/2 mv²), and potential energy (PE = mgh), along with examples and numerical problems for practice. Additionally, it covers power as the rate of doing work, defined as power = work done / time taken with its unit being watts.
Work, Energy and Power - Unit 4 - Science (Class 9)
- 1. Science Shree Shitaladevi Community Secondary School www.yepnepal.com @santoshpaudel Grade - 9 sntos29c@gmail.com
- 2. UNIT: 4 Work, Energy & Power • Work: The product of force acted on a body and displacement of a body in the direction of force is called work. It is a scalar quantity. Its SI unit is Joule and CGS unit is erg. Work(W) = Force x displacement in the direction of force W = F x d Factors affecting the Work: i) Magnitude of force ii) Direction of force iii) Displacement of the object due to force 9/13/2020www.yepnepal.com 1
- 3. Work One Joule Work: One joule work is defined as the work done when one newton (1N) force displaces an object through one meter (1m) in its direction. 1 Joule = 1N x 1m • One erg Work: One erg work is defined as the work done when one dyne (1 dyne) force displaces an object through one centimeter(1cm) in its direction. ( 1J = 107 erg) There is two types of work: i) Work against gravity ii) Work done against friction 9/13/2020www.yepnepal.com 2
- 4. Energy Ability to do work is called energy. It is scalar quantity. SI unit of energy is Joule. Types of Energy: i) Mechanical Energy ii) Electrical Energy iii) Heat Energy iv) Light Energy V) Sound Energy VI) Magnetic Energy VII) Atomic Energy VIII) Chemical Energy Mechanical Energy: Energy possessed by a body by virtue of its position or its motion is called mechanical energy. It is of two types: Kinetic energy and Potential energy. 9/13/2020www.yepnepal.com 3
- 5. Energy Potential Energy: The energy possessed by an object due to its position is called potential energy. For example: The water stored in dam, the stretched rubber, the leg lifted to kick the football, a stretched bow etc. has the potential energy. P.E. = mgh Kinetic Energy: The energy possessed by an object due to its motion is called kinetic energy. For example: pebble released from the catapult, bullet released from the gun, flowing water, wind , running man, moving vehicle etc. possess kinetic energy. K.E. = ½ mv2 9/13/2020www.yepnepal.com 4
- 6. Principle of Conservation of Energy 9/13/2020www.yepnepal.com 5 “Energy can neither be created nor be destroyed, but can be transformed from one form to another” this is called Principle of energy. Transformation of Energy: Conversion of energy from one form to another is called transformation of energy. For examples: a) On listening to the radio Chemical energy Electrical energy Sound Energy (Battery) (Wire) (Radio)
- 7. Transformation of Energy & Power 9/13/2020www.yepnepal.com 6 b) The production of electricity in hydro-stations Potential Energy Kinetic Energy Electrical Energy Light Energy (Water in dam) (Rotation of turbine) (Generator) (Bulb) Power: The rate of doing work in per unit time is called power. It is scalar quantity and SI unit is Joule per second or watt. Power = work done / time taken P= W/t P= F x d/ t (W= F x d) P= F x v ( v= d/ t)
- 8. Power 9/13/2020www.yepnepal.com 7 One Watt Power: If one Joule work is done in one second then it is called one watt power. In other words, one watt is the energy conversion at the rate of 1 Joule per second. 1 watt = 1 Joule / 1 second Bigger Units of Power are: 1 KW = 1000 W 1 MW = 1000000 W 1 Hp = 746 W
- 9. Any Questions? www.yepnepal.com @santoshpaudel sntos29c@gmail.com
- 10. Exercise based on Work Short Questions 1. When a person pushes a building, even if he gets tired, he doesn't do any work, why? 2. Is it possible that a force is acting on a body but still the work done is zero? Explain. 3. What happens when force is applied perpendicular to the displacement? 4. A coolie carrying a load on his head does no work, why? 9/13/2020www.yepnepal.com 9
- 11. Exercise based on Energy Short Questions 1. Define energy and write its SI unit. 2. What is reason behind the same SI unit of work and energy? 3. If a person sleeps for a long time, he will be in energy less condition even if he does not do any work physically why? 4. What are the factors that affect potential energy? 5. On which case, the potential energy will be more and why? 6. A piece of stone stores potential energy only when it is lifted from the surface, why? 9/13/2020www.yepnepal.com 10
- 12. Exercise based on Power Short Questions 1. What is meant by '60W written in an electric bulb’? 2. At thermal power station, one form of energy is converted into another form, Show the energy transformation. 3. The power of machine doing work fast is more, why? Clarify it. 9/13/2020www.yepnepal.com 11
- 13. Numerical Problems 1. A man applied a force of 300 N to move a table by a distance of 70 cm. What is the work done by him? (Ans: 210 J) 2. A porter carrying a load of 30kg climbs up a mountain having vertical height of 1.5km from the ground. Calculate the work done against gravity (g = 10m/s2). (Ans: 4.5 x 105J) 3. If the work done while pushing a bus through a distance of 10m is 50KJ. Calculate the force of friction of the ground. (Ans: 5000N) 4. Calculate the amount of work done by a person while taking a bag whose mass is 200kg to the top of a building of height 10m. (Use g=10 m/s2). The mass of the person is 50kg. (Ans: 2.5 x 104 J) 9/13/2020www.yepnepal.com 12
- 14. Numerical Problems 5. A stone of mass 2kg is at a height of 2m. Calculate its original energy. Now, the stone is released. What is the kinetic energy and velocity when it reaches the ground? (Ans: P.E. = 39.2 J,K.E.=39.2 J , V= 6.26m/s) 6. what happens to the mass of a body when velocity and kinetic energy of a body is decreased by 4 times? 7. A bag of rice weighs 200kg. To what height should it be raised so that its potential energy may be 9800 joules ? (g =9.8m/s2) (Ans: 5m) 8. The velocity & kinetic energy of a bullet are 200km/hr & 77.16 J respectively. Find the mass of the bullet. (Ans: m= 50 gm) 9/13/2020www.yepnepal.com 13
- 15. Numerical Problems 9. Sita weighs 40 kg. If she climbed a 3 meter high stairs in 10 seconds, what is her power? ( Ans: 120 watt) 10. Convert ; i) 5Hp into watt ii) 69 MW into Hp iii) 35KW into MW 11. A man of 50 kg mass climbs 20 steps of stair of Basantapur Durbar in one minute. If height of each step is 15cm then find his power. ( Ans: 25W ) 12. An electric motor rises 80 kg water in 3 minutes from a well 30m deep. Find the power of the motor. (Ans: 133.33 W) 13. An engine supplies 250 J of energy. If this energy is supplied to a weight of 400gm. What height can it be lifted to? ( Ans: 62.5 m) 9/13/2020www.yepnepal.com 14