1 / 85

Understanding Work and Energy in Physical Science

This lecture covers topics such as energy and work, work-energy theorem, conservation of energy, power, machines, efficiency, and sources of energy.

mreid
Download Presentation

Understanding Work and Energy in Physical Science

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 6 work & energy Physical science ~ mr.e ~ SRCS

  2. Chapter 6: WORK AND ENERGY

  3. Around 1637, Fermat wrote his Last Theorem in the margin of his copy of the Arithmetica next to Diophantus' sum-of-squares problem:[14] . it is impossible to separate a cube into two cubes, or a fourth power into two fourth powers, or in general, any power higher than the second, into two like powers. I have discovered a truly marvelous proof of this, which this margin is too narrow to contain.[15] This theorem was first conjectured by Pierre de Fermat in 1637, famously in the margin of a copy of Arithmetica where he claimed he had a proof that was too large to fit in the margin. No successful proof was published until 1995 despite the efforts of countless mathematicians during the 358 intervening years. The unsolved problem stimulated the development of algebraic number theory in the 19th century and the proof of the modularity theorem in the 20th. It is among the most famous theorems in the history of mathematics and prior to its 1995 proof was in the Guinness Book of World Records for "most difficult math problems".

  4. This lecture will help you understand: • Energy and Work • Work-Energy Theorem • Conservation of Energy • Power • Machines • Efficiency • Sources of Energy Rube Goldberg Napkin Machine - YouTube Honda Commercial Rube Goldberg

  5. Work Augie March - The Cold Acre - YouTube

  6. Work Work • defined as the product of force exerted on an object and the distance the object moves (in the same direction as the force) • is done only when the force succeeds in moving the body it acts upon • equation: work = force  distance W = Fd

  7. Work Two things must be present where work is done: • application of F • movement of something by that force Work done on the barbell is the average force multiplied by the distance through which the barbell is lifted.

  8. Work Here’s a question: According to the scientific definition of work, am I actually accomplishing any? What are the Units for Work? Best way to figure this out? Formula! W = F d W = N x m (read: “Newton-meters”)

  9. Work CHECK YOUR NEIGHBOR If you push against a stationary brick wall for several minutes, you do no work on the wall. A. True. • False. • It depends. • Which prison system do I belong to?

  10. Work CHECK YOUR ANSWER If you push against a stationary brick wall for several minutes, you do no work on the wall A. True. • False. • It depends. • Which prison system do I belong to? Explanation: You may do work on your muscles, but not on the wall.

  11. Work actually accomplishing any? Back to the Units for Work… W = F d W = N x m A Newton-meter is also known as a joule (J)

  12. Work One joule in everyday life is approximately: • the energy required to lift a quarter pounder patty (w/out cheese)…what distance?

  13. Work One joule in everyday life is approximately: • the energy required to lift a small apple one metre straight up. (A mass of about 102 g ) • the energy released when that same apple falls one metre to the ground. • the energy released as heat by a person at rest, every 17 seconds. • the kinetic energy of a 50 kg human moving very slowly (0.2 m/s)—20cm/s. • the kinetic energy of a tennis ball moving at 23 km/h (14 mph).

  14. Work More joules in everyday life… Kilojoule • One kilojoule per second (1000 watts) is approximately the amount of solar radiation received by one square meter of the Earth in full daylight.[8]

  15. Work More joules in everyday life… Terajoule • The terajoule (TJ) is equal to one trillion (1012) joules. About 63 terajoules were released by the atomic bomb that exploded over Hiroshima.[10] • The International Space Station, at completion, with a mass of 450,000kg and orbital velocity of 7.7 km/s,[11] will have a kinetic energy of roughly 13 terajoules.

  16. Work More joules in everyday life… Petajoule • The petajoule (PJ) is equal to 1015 joules. 210 PJ is equivalent to about 50 megatons of TNT. This is the amount of energy released by the Tsar Bomba, the largest man-made nuclear explosion ever.

  17. Work More joules in everyday life… Exajoule • The exajoule (EJ) is equal to 1018 joules. The 2011 Tōhoku earthquake and tsunami in Japan manifested 1.41 EJ of energy • in the United States used per year is roughly 94 EJ.

  18. Work More joules in everyday life… Zettajoule • The zettajoule (ZJ) is equal to 1021 joules. Annual global energy consumption is approximately 0.5 ZJ

  19. Work More joules in everyday life… Yottajoule • The yottajoule (YJ) is equal to 1024 joules. This is approximately the amount of energy required to heat the entire volume of water on Earth by 1 °Celsius.

  20. Work The quantity of work done is equal to the amount of force  the distance moved in the direction in which the force acts. Work falls into two categories: • work done against another force Powerlifter Shane Hamman • work done to change the speed of an object Spiderman 2 (2004) Peter Stops The Train! http://www.youtube.com/watch?v=xAYAvLSwQGw

  21. Work CHECK YOUR NEIGHBOR Work is done in lifting a barbell. How much work is done in lifting a twice-as-heavy barbell the same distance? A. Twice as much. • Half as much. • The same. • Depends on the speed of the lift.

  22. Work CHECK YOUR ANSWER Work is done in lifting a barbell. How much work is done in lifting a twice-as-heavy barbell the same distance? A. Twice as much. • Half as much. • The same. • Depends on the speed of the lift. Explanation: This is in accord with work = force  distance. Twice the force for the same distance means twice the work done on the barbell.

  23. Work CHECK YOUR NEIGHBOR You do work when pushing a cart. If you push the cart twice as far with the same constant force, then the work you do is A. less than twice as much. • twice as much. • more than twice as much. • zero.

  24. Work CHECK YOUR ANSWER You do work when pushing a cart. If you push the cart twice as far with the same constant force, then the work you do is A. less than twice as much. • twice as much. • more than twice as much. • zero.

  25. Power Define WORK again… Which aspect is left out? Here’s a hint (riddle): This thing all things devours:Birds, beasts, trees, flowers;Gnaws iron, bites steel;Grinds hard stones to meal;Slays king, ruins town,And beats high mountain down.  

  26. Power Which accomplishes more WORK…Does it make a difference if I walk or run upstairs? Why not? Then why are you more tired when running up the stairs as opposed to merely walking up the stairs? Does one require more E? Let’s see…

  27. Power Power is the rate at which work is performed or energy is converted (AKA: measure of how fast work is done) What are the Units for Power? What is the formula for Power? P = Work accomplished/t taken  W/t P = J/s Which uses more E? Doing something fast or doing the same thing slow? Why?

  28. Power How much energy is required to lift a quarter pounder patty (w/out cheese) one m? How about lifting the same patty one m in 1 second? If lifting with 1 N of F a distance of 1 m expends 1 J of E…1 J expended in 1 second = 1 Watt I converted the steam engine from a “prime mover of marginal efficiency into the mechanical workhorse of the Industrial Revolution" P=W/t P=Fxd/t P=Nxm/s P = J/s 1 J/s = 1 watt

  29. Power 1 horsepower is equivalent to 746 watts. So if you took a 1-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts. 5.5 Hp The development of the steam engine provided a reason to compare the output of horses with that of the engines that could replace them. • "Watt found by experiment in 1782 that a 'brewery horse' was able to produce 32,400 foot-pounds per minute." James Watt and Matthew Boulton standardized that figure at 33,000 the next year.[7] • Most observers familiar with horses and their capabilities estimate that Watt was either a bit optimistic or intended to underpromise and overdeliver; few horses can maintain that effort for long. Regardless, comparison with a horse proved to be an enduring marketing tool. ~12 hp One unit of mechanical horsepower is approximately equivalent to 745.7 watts.

  30. Power A healthy human can produce about 1.2 hp briefly and sustain about 0.1 hp indefinitely; trained athletes can manage up to about 2.5 hp briefly and 0.3 hp for a period of several hours… Surfer Rides 90 Foot Wave, Sets New...

  31. Power CHECK YOUR NEIGHBOR A job can be done slowly or quickly. Both may require the same amount of work, but different amounts of A. energy. • momentum. • power. • impulse.

  32. Power CHECK YOUR ANSWER A job can be done slowly or quickly. Both may require the same amount of work, but different amounts of A. energy. • momentum. • power. • impulse. Comment: Power is the rate at which work is done.

  33. Energy This is an example of stored E (Potential E)…for our purposes, definition has a mechanical E spin… Energy • defined as that which produces changes in matter • (AKA the ability to do what?) Effects of Mechanical energy observed only when • it is being transferred from one place to another or • it is being transformed from one form to another Both work and energy are measured in joules.

  34. Mechanical Energy Two Types • E due to an object’s POSITION or • E due to its MOVEMENT What do we call E of position? What do we call E of motion?

  35. Potential Energy Example: potential energy of 10-N ball is the same in all 3 cases because work done in elevating it is the same. Why?

  36. Potential Energy Potential Energy is defined as stored energy due to position, shape, or state. In its stored state, energy has the potential for doing work. Examples: Drawn bow Stretched rubber band Raised ram of a pile driver…..others? How did the object get that E?

  37. Gravitational Potential Energy The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in raising it. Work done equals force required to move it upward  the vertical distance moved (W = Fd). The upward force when moved at constant velocity is the weight, mg, of the object. So the work done in lifting it through height h is the product mgh.

  38. Gravitational Potential Energy Equation for gravitational potential energy: PE = weight  height or PE = mgh Gravitational potential energy examples: Water in an elevated reservoir The elevated ram of a pile driver How derived? 1st formula? F=ma so F= mg Substitute: PE = Fxd = mgxh or mgh Try your hand at the text questions in the blue box on p. 88…

  39. Potential Energy CHECK YOUR NEIGHBOR Does a car hoisted for repairs in a service station have increased potential energy relative to the floor? A. Yes. • No. • Sometimes. • Not enough information.

  40. Potential Energy CHECK YOUR ANSWER Does a car hoisted for repairs in a service station have increased potential energy relative to the floor? A. Yes. • No. • Sometimes. • Not enough information. Comment: And if the car were twice as heavy, its increase in potential energy would be twice as much.

  41. Kinetic Energy …is defined as the energy of a moving body; that is, the KE of an object is equal to the W required to bring it to that speed from rest or the amount of W that object can do while being brought to rest. Equation for kinetic energy: Kinetic energy = 1/2 mass  speed2 or KE = 1/2mv2 small changes in speed  large changes in KE (In fact if you double the speed of an object, you _____ the KE? Why?

  42. Kinetic Energy KE = 1/2mv2 How did we get this formula? Consider: F = ma (mult. by d) F d = mad (d= ½ at2) Fd = ma (½ at2) Fd = ½ m a2t2 (a=v/t or v=at) Fd = ½ mv2 KE = ½ mv2 Try your hand at the text questions in the blue box on p. 89… The story of kinetic and potential energy

  43. Work-Energy Theorem • Applies to decreasing speed • reducing the speed of an object or bringing it to a halt Example:Applying the brakes to slow a moving car. Work is done on it (the friction force supplied by the brakes  distance). d increased by what factor? Why? Do you see a correlation between the data and the formula? KE = ½ mv2

  44. The Work-Energy Theorem CHECK YOUR NEIGHBOR The work done in braking a moving car to a stop is the force of tire friction  stopping distance. If the initial speed of the car is doubled, the stopping distance is A. actually less. • about the same. • twice. • None of the above.

  45. The Work-Energy Theorem CHECK YOUR ANSWER The work done in braking a moving car to a stop is the force of tire friction  stopping distance. If the initial speed of the car is doubled, the stopping distance is A. actually less. • about the same. • twice. • None of the above. Explanation: Twice the speed means four times the kinetic energy and four times the stopping distance. Try your hand at the text questions in the blue box on p. 90…

  46. Work-Energy Theorem When work is done on an object to change its KE, the amount of work done is equal to the change in KE. Equation for work-energy theorem: Net work = change in KE • If there is no change in object’s energy, then no work is done on the object. • Applies to potential energy: For a barbell held stationary, no further work is done no further change in energy. • Applies to decreasing energy: The more kinetic energy something has  the more work is required to slow it or stop it

  47. The Work-Energy Theorem CHECK YOUR NEIGHBOR Consider a problem that asks for the distance a fast-moving crate slides across a factory floor in coming to a stop. The most useful equation for solving this problem is A. F = ma. B. Ft = mv. C. KE = 1/2mv2. • Fd = 1/2mv2.

  48. The Work-Energy Theorem CHECK YOUR ANSWER Consider a problem that asks for the distance a fast-moving crate slides across a factory floor in coming to a stop. The most useful equation for solving this problem is A. F = ma. B. Ft = mv C. KE = 1/2mv2. D. Fd = 1/2mv2. Comment: The work-energy theorem is the physicist’s favorite starting point for solving many motion-related problems.

  49. Conservation of Energy with a Simple Pendulum Conservation of Energy Conservation of Energy Demo with a Bowling Ball Example: energy transforms without net loss or net gain in the operation of a pile driver How do we know? Consider: Impulse = D p F t= D mv (divide by 2) F t= D mv 2 2 (mult. By v) v x ½ Ft = ½ mv x v ½ Ftv = ½ mv 2 ½ Ft x d/t = ½ mv 2 (parse v; cancel t’s) ½ Fd= ½ mv 2 (mult. by 2) Fd = mv2 (fill in units) N x m = kg x m2/s2 (parse unit) N x m = kg x m/s2 x m (Subst. concepts) Fd = Fd Work = Work

More Related