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What is energy?

What is energy?. What is Energy? Energy is the ability to cause change. Forms of Energy: Energy is found in different forms, such as light, heat, sound and motion . There are many forms of energy, but they can all be put into two categories: kinetic and potential. Thermal Energy.

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What is energy?

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  1. What is energy?

  2. What is Energy? • Energy is the ability to cause change. • Forms of Energy: Energy is found in different forms, such as light, heat, sound and motion. • There are many forms of energy, but they can all be put into two categories: kinetic and potential.

  3. Thermal Energy

  4. Geothermal energy

  5. Potential Energy • An object can store energy as the result of its position. This stored energy of position is referred to as potential energy.

  6. Gravitational Potential Energy • Gravitational potential energy is the energy stored in an object as the result of gravitational attraction of the Earth for the object. • There is a direct relation between gravitational potential energy and the mass of an object. • More massive objects have greater gravitational potential energy. There is also a direct relation between gravitational potential energy and the height of an object. The higher that an object is elevated, the greater the gravitational potential energy is.

  7. These relationships are expressed by the following equation: Gravitational potential Energy= force of gravity* Height GPE = m *g * h m= mass of the object, h : height of the object and g: acceleration due gravity (9.8 m/s2 on Earth).

  8. If the velocity and time for a free-falling object being dropped from a position of rest were tabulated, then one would note the following pattern. Why is g= 9.8?

  9. Calculate acceleration for each time interval.

  10. Questions: • 1-A ball of mass 1 kg is thrown up to a height of 12 meters. Calculate the increase in its potential energy. • Solution:GEP  = mghGEP  = (1)(9.8)(12)GEP  = 117.6 J

  11. 2-How far above the surface of the earth would the same ball (1 kg) have to be thrown for its potential energy to increase to 1 MJ? • Solution:GPE   =   mgh 106    = (1)(9.8) hh  = 106 / 9.8h  = 102 040.82 m

  12. Kinetic Energy

  13. Kinetic energy is the energy of motion. An object which has motion - whether it is vertical or horizontal motion - has kinetic energy. There are many forms of kinetic energy - vibration (the energy due to vibration motion), rotational (the energy due to rotational motion), and translational (the energy due to motion from one location to another). To keep matters simple, we will focus upon translational kinetic energy. The amount of translational kinetic energy which an object has depends upon two variables: the mass (m) of the object and the speed (v) of the object.

  14. The following equation is used to represent the kinetic energy where m = mass of object; v = speed of object • The unit of energy is Jules • This equation reveals that the kinetic energy of an object is directly proportional to the square of its speed.

  15. Questions: 1. Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s. KE = 0.5*m*v2 KE = (0.5) * (625 kg) * (18.3 m/s)2 KE = 1.05 x105 Joules

  16. 2. If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy? If the speed is doubled, then the KE is quadrupled. Thus, KE = 4 * (1.04653 x 105 J) = 4.19 x 105 Joules.

  17. 3. Missy Diwater, the former platform diver for the Ringling Brother's Circus, had a kinetic energy of 12000 J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed? • KE = 0.5*m*v2 • 12 000 J = (0.5) * (40 kg) * v2 • 300 J = (0.5) * v2 • 600 J = v2 • v = 24.5 m/s

  18. 4. A 900-kg compact car moving at 60 mi/hr has approximately 320 000 Joules of kinetic energy. Estimate its new kinetic energy if it is moving at 30 mi/hr. • KE = 80 000 J • The KE is directly related to the square of the speed. If the speed is reduced by a factor of 2 (as in from 60 mi/hr to 30 mi/hr) then the KE will be reduced by a factor of 4. Thus, the new KE is (320 000 J)/4 or 80 000 J.

  19. The Law of Conservation of Energy To scientists, conservation of energy is not saving energy. The law of conservation of energy says that energy is neither created nor destroyed. When we use energy, it doesn’t disappear. We change it from one form of energy into another.

  20. Example 2: Oil burns to make heat --> Heat boils water --> Water turns to steam --> Steam pressure turns a turbine --> Turbine turns an electric generator --> Generator produces electricity --> Electricity powers light bulbs --> Light bulbs give off light and heat

  21. The mass of the ball = 10kg The height, h = 0.2m The acceleration due to gravity, g = 9.8 m/s^2 • What is the potential energy of the ball at position 1? PE = 19.6J (J = Joules, unit of energy)

  22. Position 1 of the ball is where the Potential Energy (PE) =19.6, while the Kinetic Energy (KE) = 0 • As ball 1 is approaching position 2 ,PE is decreasing while the KE is increasing • At exactly halfway between position 1&2 PE=KE • The Position 2 is where the Kinetic Energy is at its maximum while the Potential Energy = 0. At this point, theoretically, all the PE has transformed into KE Therefore now the KE = 19.6J while the PE = 0. • The position 3 of the ball is where the Potential Energy (PE) is once again at its maximum and the Kinetic Energy (KE) = 0.

  23. Example 3: How much kinetic energy a 2 Kg ball will have when it bounces on the floor after being thrown to a height of 4 m above the floor. (ignore friction) KE Bottom =GPE top = mgh = (2) (9.8) (4)= 78 J

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