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# Work and energy

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### Work and energy

1. 1. Introduction Consider the following day-to-day activities which we consider as work: Reading, speaking, singing, writing, thinking etc. We require energy to perform these activities, which we
2. 2. Even if we push a wall with the maximum force that we can apply, the wall will not move. It will be interesting for us to note that even in this case, we are not doing any work at all! Work is not done in all the above activities because there
3. 3. Scientifically, work is defined as the work done by a force that causes a displacement in an object. If we push a book placed on a table with a force, then it will move to a certain distance. Scientifically, we will say that some work has been done on the book. In this case, work is done
4. 4. If we lift the book to a certain height, then a force is exerted against gravity, which displaces the book to a certain height. Hence, we can say that work is done on the book against the
5. 5. Work done Whenever displacement is brought in the line of force applied we call it as work done. Mathematically work done is given a symbol W and it is defined as the product of force(F) and
6. 6. There could be four different cases according to the force and displacement relationship. If: +Force +Displacement = +Work -Force -Displacement = +Work -Force +Displacement = -Work +Force -Displacement = -Work
7. 7. Work Done By a Constant Force A wooden block is kept on a table. When a force of magnitude F acts on the block, it gets displaced through a distance S in the direction of the applied force, as shown in the given figure.
8. 8. The magnitude of work done is given by the product of force (F) and displacement (S). Let W be the work done on the block. ∴ Work = Force × Displacement W= F × S
9. 9. Unit of Work To obtain the unit of work, we substitute the SI units of force, i.e. N, and distance, i.e. m, in the equation of work. W = N × m = Nm Hence, the unit of work is Nm. In the honor of physicist James P. Joule, the SI unit of work is written as Joule (J). Hence, 1 J = 1 Nm 1 Joule is defined as the
10. 10. Work done against gravity When force is applied on an object in order to lift it above the ground, it is said that work is done against the force of gravity. Assume that a constant force of magnitude F is applied on a block of mass m to lift it to a
11. 11. In this case, the work done by the force against gravity is given by the product of the weight of the block and the height through which it is lifted above the ground. Work done = Weight × Height W = mg × h
12. 12. Negative work If the force acts opposite to the direction of displacement, then the WORK done will be negative i.e. W=F x (-s) or (-F x s). Here, the directions of displacement (S) and applied force (F) are exactly opposite to each other. Suppose, a soccer player moves
13. 13. Zero Work When a body moves through a distance at right angle to the direction of force, the work done by the force on the body is zero. A book kept on a table moves from point A to point B through a distance S. In this case, the work done on the book by
14. 14. Energy The world requires a lot of energy. To satisfy this demand, we have natural energy sources such as the sun, wind, water at a height, tides, etc. We also have artificial energy sources such as
15. 15. Forms of energy Some forms of energy are (i) Light (ii) Sound (iii) Heat (iv) Mechanical (v) Electrical (vi) Chemical (vii) Nuclear
16. 16. Mechanical energy It is the form of energy possessed by an object that has the potential to do work. It is caused by the motion or the position and configuration of the object. Mechanical energy is of two types. (i) Kinetic energy (caused by
17. 17. Kinetic energy Energy stored into an object due to its motion. A moving arrow can be embedded into an object. Hence, it is said that the arrow possesses kinetic energy. The elastic string of a catapult is stretched to throw a stone. The
18. 18. A stone dropped from a height has the capability to create a depression in wet ground. Hence, the dropped stone has some amount of kinetic energy. A fired bullet is embedded in a wall or wooden block. Hence, it is said that a moving bullet possesses kinetic energy.
19. 19. Formula for kinetic energy Kinetic energy of a moving body is equal to the work required to change its velocity from u to v. Let a body of mass m be moving with a uniform velocity u. Let an external force be applied on it so that it displaces a distance s and its velocity
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23. 23. Kinetic energy of a body is directly proportional to: (i) Its mass (m) (ii) The square of its velocity (v2) It is the kinetic energy of the wind that is used in windmills to generate electricity.
24. 24. Potential Energy There are mainly two types of potential energy: Potential energy possessed by a body by virtue of its configuration is known as elastic potential energy Potential energy possessed by a body by virtue of its position with respect to the ground is known as gravitational potential
25. 25. Potential energy of an object at an height……… Any object located at a height with respect to a certain reference level is said to possess energy called gravitational potential energy. This energy depends on this reference level (sometimes
26. 26. When a ball is taken to the top floor from the ground floor, it acquires some gravitational potential energy. When this ball is dropped from a height h1 on the top floor, the zero level is the top floor itself. When the ball is dropped from a height h2 on the ground, the zero level is the ground. Since the distance covered by
27. 27. Hence, we conclude from the above discussion that potential energy stored in a body is directly proportional to its height with
28. 28. Formula for gravitational potential energy Consider an object of mass 'm', raised through a height 'h' above the earth's surface. The work done against gravity gets stored in the object as its Potential Energy (Gravitational Potential
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30. 30. Law Of Conservation Of Energy Energy can neither be created nor destroyed. It can only be transformed from one form to another. In other words, the total amount of energy in a system always remains constant. For example, in a burning
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33. 33. The sum of kinetic energy and potential energy of an object is its total mechanical energy. We find that during the free fall of the object, the decrease in potential energy, at any point in its path, appears as an equal amount of increase in kinetic energy. (Here the effect of
34. 34. Power 
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36. 36. Commercial Unit Of Energy Joule is a very small unit of energy. Therefore, we use bigger units of energy for commercial purposes. This commercial unit of energy is kilowatt-hour (kWh). We define kilowatthour as the amount of energy consumed when an electrical appliance of
37. 37. The relation between joule and kilowatt-hour is given by 1kWh = 3600000 Ws = 3.6 ×106 J The amount of electrical energy consumed in our house is expressed in
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