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Energy: Work and Power

Energy: Work and Power. Mechanical energy. Mechanical energy. Radiant energy. Mechanical energy includes energy associated with motion and position. Examples of mechanical energy include: kinetic energy gravitational potential energy elastic potential energy. Nuclear energy.

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Energy: Work and Power

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  1. Energy: Work and Power

  2. Mechanical energy Mechanical energy Radiant energy • Mechanical energy includes energy associated with motion and position. • Examples of mechanical energy include: • kinetic energy • gravitational potential energy • elastic potential energy Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  3. Radiant energy Mechanical energy Radiant energy • Radiant energy includes all forms of electromagnetic radiation. Examples include: • visible light • infrared radiation • ultraviolet light • microwaves • radio waves • x-rays Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  4. Nuclear energy Mechanical energy Radiant energy Nuclear energy comes from the forces that hold the atomic nucleus together. Any change in an atom’s nucleus absorbs or releases nuclear energy. Nuclear energy is the energy source for the Sun and stars. Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  5. Electrical energy Mechanical energy Radiant energy Electrical energy is carried by electric currents. Electrical energy is clean, moves easily through wires, and is a key energy resource in human technology. Electrical energy must be derived from other forms of energy, such as nuclear or chemical energy. Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  6. Chemical energy Mechanical energy Radiant energy Chemical energy is energy stored in the bonds between atoms. When chemical bonds are changed through chemical reactions, chemical energy is released. Chemical energy is found in foods and fuels. Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  7. Thermal energy Mechanical energy Radiant energy Thermal energy is energy due to temperature. An object at a higher temperature has more thermal energy than if it was at a lower temperature. The thermal energy also depends on the mass and material of the object. Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  8. Fluid pressure Mechanical energy Radiant energy Fluid pressure in gases and liquids creates a form of stored energy. The high-pressure air in a bicycle tire has more energy (per cubic centimeter) than the lower-pressure air outside. Nuclear energy Electrical energy Chemical energy Thermal energy Internal energy of gases

  9. The meaning of energy Mechanical energy Radiant energy Nuclear energy Energy is the mediator of all physical changes, such as changes in speed, height, temperature, or chemical composition. Energy is measured in joules. Electrical energy Chemical energy Thermal energy Internal energy of gases

  10. Kinetic energy is energy due to motion. Energy of motion KE It depends on the mass of an object, and its speed.

  11. Kinetic energy is energy due to motion. Energy of motion KE A 1 liter water bottle (1 kg) moving at 1 m/s (2.4 mph) has a kinetic energy of . . .

  12. Kinetic energy is energy due to motion. Energy of motion KE A 1 liter water bottle (1 kg) moving at 1 m/s (2.4 mph) has a kinetic energy of 0.5 joules. KE

  13. Engaging with the concepts What is the speed of a 2000 kg car that has a kinetic energy of 500,000 joules? KE

  14. Engaging with the concepts What is the speed of a 2000 kg car that has a kinetic energy of 500,000 joules? 22.36 m/s.

  15. Calculating kinetic energy KE The kinetic energy increases as the square of the speed.

  16. Calculating kinetic energy KE The kinetic energy increases as the square of the speed. If the speed triples, how does the kinetic energy change?

  17. Calculating kinetic energy KE The kinetic energy increases as the square of the speed. If the speed triples, how does the kinetic energy change? 32 = 9 The kinetic energy increases by a factor of 9. KE

  18. Kinetic energy comes from work

  19. Gravitational potential energy If the mass of the container increases, its potential energy will also increase. If the height of the container increases, its potential energy will also increase.

  20. Gravitational potential energy The gravitational potential energy of an object is . . .

  21. Gravitational potential energy How can you give an object gravitational potential energy? Gravitational potential energy comes from work done against gravity ... … such as the work you do when you lift this bottle of water.

  22. Athletics and energy How much energy does it take to raise a 70 kg (154 lb) person one meter off the ground?

  23. Athletics and energy How much energy does it take to raise a 70 kg (154 lb) person one meter off the ground? PE This is a good reference point. It takes 500 to 1,000 joules for a very athletic jump.

  24. Does the path matter? • A set of identical twins wants to get to the top of a mountain. • One twin hikes up a winding trail. • The second twin takes the secret elevator straight to the top. • Which twin has the greatest potential energy at the top?

  25. Path independence The twins have the SAME potential energy at the top. It doesn’t matter HOW they gained height. Changes in potential energy are independent of the path taken. PE

  26. Work • W = Fd • How much work is done pushing a box 10m with a force of 20N? • How much work is done lifting a 10kg box 2m?

  27. Brainstorm A 1200 kg sports car accelerates from 0 to 60 mph (27 m/s) in 5.0 seconds. A typical car of equal mass takes 8.0 seconds to do the same thing. So what’s the difference?

  28. Power The difference is power. Both cars convert the chemical energy of their gasoline into the same amount of kinetic energy . . . . . . but the sports car converts energy at a faster rate. It converts more energy every second. The sports car has greater power.

  29. Any event in which energy is transformed involves power. Power is typically measured in watts. Power

  30. An elephant versus an ant An elephant lifts a 300 kg log to a height of 3.0 meters in a time of four seconds. An ant can lift several times its own body weight in a time of two seconds. Which one is more powerful?

  31. An elephant versus an ant The elephant is more powerful, of course! The elephant transforms more chemical energy (from her cells) into gravitational potential energy each second.

  32. Power is a rate Power is the rate at which work is done. Lifting a ball quickly requires more power than lifting it slowly. Power = Work time

  33. Test your knowledge Nala does twice as much work as Mateo in the same amount of time. How much power does she have, compared to Mateo?

  34. Test your knowledge Nala does twice as much work as Mateo in the same amount of time. How much power does she have, compared to Mateo? Twice as much power. Alonzo and Irina do the same amount of work, but Irina takes twice as long. How much power does Alonzo have, compared to Irina?

  35. Test your knowledge Nala does twice as much work as Mateo in the same amount of time. How much power does she have, compared to Mateo? Twice as much power. Alonzo and Irina do the same amount of work, but Irina takes twice as long. How much power does Alonzo have, compared to Irina? Twice as much power.

  36. Calculating power An elephant lifts a 300 kg log to a height of 3.0 meters in a time of 4.0 seconds. How much power is required? What is the first step?

  37. Calculating power An elephant lifts a 300 kg log to a height of 3.0 meters in a time of 4.0 seconds. How much power is required? First, find the energy transformed. The elephant converts energy from her cells into the potential energy of the log.

  38. Calculating power An elephant lifts a 300 kg log to a height of 3.0 meters in a time of 4.0 seconds. How much power is required? First, find the energy transformed. The elephant converts energy from her cells into the potential energy of the log. Then calculate the power:

  39. Calculating power An elephant lifts a 300 kg log to a height of 3.0 meters in a time of 4.0 seconds. How much power is required? First, find the energy transformed. The elephant converts energy from her cells into the potential energy of the log. Then calculate the power:

  40. Units for power • The average power of a sports car is 84,100 watts. • This is such a large number. How else can it be expressed? • There are two other ways: • Convert it to kilowatts: • What other unit often used for the power of a car?

  41. Units for power What other unit often used for the power of a car? The horsepower! One horsepower equals 746 watts. For the sports car: This is the average power output for the entire event. The power needed at each instant keeps increasing as the velocity increases.

  42. Energy The energy transformed in an event can be computed if the power is known. Start with the power equation. Multiply both sides byΔt. Cancel the Δt’s on the right side and swap the sides.

  43. Calculating energy A toaster oven operating at 800 watts takes one and a half minutes to toast a bagel. How much energy is this?

  44. Calculating energy A toaster oven operating at 800 watts takes one and a half minutes to toast a bagel. How much energy is this?

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