Work energy &power
- 2. Introduction In everyday life, we see some objects at rest and others in motion. We often perceive that an object to be in motion when its position changes with time. The phenomena of sunrise, sunset & changing of seasons due to motion of the earth. Work is an activity performed by the still bodies or bodies in motion. All living beings need food. Living beings have to perform several basic activities to survive. The energy for this processes comes from the food. More energy is needed for singing, dancing, running, writing, thinking & cycling. Animals also need energy to do some activities. Machines also need petrol & diesel for their working.
- 3. . Work Units Of Work Some Examples Of Works In Our Daily Life Energy Units Of Energy Forms Of Energy Law of conservation of energy Power Units Of Power Commercial units of energy
- 4. WORK All living beings need to take food because all of them have to perform several activities to live. These activities are known as ‘Life Processes’. These processes require energy which comes from food. The requirement of energy depends upon the type of work.
- 5. Work defined in Science Work is defined (in calculus terms) as the integral of the force over a distance of displacement. Work done on an object is defined as the magnitude of force multiplied by its distance moved by the object in the direction of applied force. Work done = Force х Displacement W = F х S
- 6. Units of work The SI units for work are the joule (J) . Newton-meter (N * m), from the function W= F * s where W is work, F is force, and s is the displacement. The joule is also the SI unit of energy.
- 7. Some examples of work in our daily life Work in day-to-day life is different to the work we use in science. Day-to-day works: • Working hard to prepare for examinations • Reading books • Drawing diagrams • Playing in a field • Talking with friends • Watching a movie • Attending functions • Humming a tune
- 8. Some examples of work s in science Works in science: • Standing still with a heavy load on our heads • Climbing the steps of a staircase • Pulling a trolley to some distance • Lifting a book to a height • Push a pebble lying on the surface • Bullock pulling a cart
- 9. Work In the above examples we have observed the difference in day -to-day works & works in science. In some of the examples the work is not done. Even though we are using energy work is not done. Work is energy transformed by a force. There are two conditions for the work to be done. They are 1. A force should act on object 2. The object must be displaced
- 10. Work done by a constant force • To understand this, we shall first consider the case when the force is acting in the direction of displacement. • Let a constant force, F act on an object. Let the object be displaced through a distance, s in the direction of the force. Let W be the work done. We define work to be equal to the product of the force and displacement. Work done = force* displacement W =Fs
- 11. Work done by a constant force A simple example: Let a constant force, F act on an object. Let the object be displaced through a distance, s in the direction of the force & the object stopped after a displacement. Let W be the work done. We define work to be equal to the product of the force and displacement. In this case F is taken as negative. The work done by the force is W = (-F ) * S or W = F * (-S)
- 12. Energy Energy is defined as the capacity to do work. The word energy is very often used in our daily life, but in science we give it a definite and precise meaning. The demand for energy is ever increasing. The Sun is the biggest source of energy to us. Many of our energy resources are derived from the Sun. We can also get energy from the nuclei of atoms, the interior of the earth, and the tides.
- 13. Examples of energy • Let us consider the following examples :when a fast moving cricket ball hits a stationary wicket, the wicket is thrown away. • When a raised hammer falls on a nail placed on apiece of wood, it drives the nail into the wood. • When a balloon filled with air is pressed we see a change in its shape. However, if we press the balloon hard, it can even explode producing a blasting sound.
- 14. Forms of Energy In the world we live in provides energy in many different forms. The various forms include mechanical energy(potential energy + kinetic energy), heat energy, chemical energy, electrical energy and light energy. James Prescott Joule 1818 --- 1889 James Prescott Joule was an outstanding British physicist. He is best known for his research in electricity and thermodynamics. Amongst other things, he formulated a law for the heating effect of electric current. He also verified experimentally the law of conservation of energy and discovered the value of the mechanical equivalent of heat. The unit of energy and work called joule is named after him.
- 15. Kinetic Energy & Potential Energy Kinetic energy is energy possessed by a body by virtue of its movement. kinetic energy of an object is relative to the state of other objects in its environment. Potential energy is the energy possessed by a body by virtue of its position or state. potential energy is completely independent of its environment. Hence the acceleration of an object is not evident in the movement of one object, where other objects in the same environment are also in motion. For example, a bullet whizzing past a person who is standing possesses kinetic energy, but the bullet has no kinetic energy with respect to a train moving alongside.
- 16. Kinetic energy Vs Potential Energy Kinetic Energy The energy of a body or a system with respect to the motion of the body or of the particles in the system. Kinetic energy of an object is relative other moving and stationary objects in its immediate environment. Kinetic energy can be transferred from one moving object to another, say, in collisions. Flowing water, such as when falling from a waterfall. Joule (J). Potential Energy Potential Energy is the stored energy in an object or system because of its position or configuration. Potential energy is not relative to the environment of an object. Potential energy cannot be transferred. Water at the top of a waterfall, before the precipice. Joule (J)
- 17. Law of conservation of energy o The law of conservation of energy is one of the basic laws of physics. o The law of conservation energy states that ‘In a closed system, i.e., a system that isolated from its surroundings, the total energy of the system is conserved’. o In SI units, energy has units of Joules. 1 Joule = 1 kgms. o Whenever energy gets transformed, the total energy remains unchanged. This is the consequence of law of conservation of energy. o According to this law, energy can only be converted from one form to another; it can neither be created nor destroyed. o The total energy before and after the transformation remains the same.
- 18. Law of Conservation of energy A Simple example: Let an object of mass, m be made to fall freely from a height, h. At the start, the potential energy is mgh and the kinetic energy is zero. It is zero because its velocity is also zero. The total energy of the object is thus mgh. As it falls, its potential energy will change into kinetic energy.If v is the velocity of the object at a given instant, the kinetic energy would be 1/2mv². As the fall continues, the potential energy would decrease while the kinetic energy would increase. That is potential energy + kinetic energy =constant or mgh+1/2mv²=constant The sum of kinetic energy and potential energy of an object is its total mechanical energy
- 19. Power or Rate of doing work • Power is defined as the rate of doing work or rate of transfer of energy. If ‘W’ joule of work is done in ‘t’ seconds, power P = work done/time taken =w/t • The unit of power is watt. 1watt of the power machine or an agent that does work at the rate of 1joule per second. 1000watts = 1kilowat 1000kilowatt =1 megawatt 1kw = 1000j sˉ¹
- 20. Power or Rate of doing work WORK AND POWER • When you walk a mile, your motive force is displacing your body, which is measured as the work done. • When you run the same mile, you are doing the same amount of work but in less time. The runner has a higher power rating than the walker, putting out more watts. A car with 80 horsepower can produce faster acceleration than a car with 40 horsepower. In the end, both cars are going 60 miles per hour, but the 80-hp engine can reach that speed faster. • In the race between the tortoise and the hare, the hare had more power and accelerated faster, but the tortoise did the same work and covered the same distance in a much longer time. The tortoise showed less power. • AVERAGE POWER • When discussing power, people are usually referring to average power, Pavg. It is the amount of work done in a period of time (ΔW/Δt) or the amount of energy transferred in a period of time (ΔE/Δt). • INSTANTANEOUS POWER • What is the power at a specific time? When the unit of time approaches zero, calculus is needed to derive an answer, but it is approximated by force times speed
- 21. Power or Rate of doing work The power agent may vary with time. This means that the agent may be doing work at different rates at different intervals of time. Therefore the concept of average power is useful. We obtain average power by dividing the total energy consumed by the total time taken.
- 22. Commercial units of energy • The unit joule is too small and hence is inconvenient to express large quantities of energy. We use bigger unit of energy called kilowatt hour (kW h). • Let us say we have a machine that uses 1000j of energy every second. If this machine is used continuously for one hour, it will consume 1kW of energy. Thus, 1kW h is the energy used in one hour at the rate of 1000j sˉ¹ (or 1kW). 1kW h =1kW *1h =1000W *3600s =3600000j 1kW h = 3.6 *10‘ j. • The energy used in households, industries, and commercial establishments are usually expressed in kilowatt hour. For example electrical energy used during a month is expressed in terms of ‘units’. Here, 1 unit means 1kilowatt hour.