What you should know about ENERGY

1. What you should know about ENERGY Ch07 - Energy - Revised: 2/21/10

2. This lecture will help you understand: Energy Work Power Mechanical Energy : Potential and Kinetic Conservation of Energy Ch07 - Energy - Revised: 2/21/10

3. Energy What is energy? The ability to do work. Energy • Mover of things • Observed when it is being transferred or being transformed • Not spooky life force stuff. There is no such thing as good or bad energy, (Sorry Cesar Millan). Ch07 - Energy - Revised: 2/21/10

4. Circular definition of work/ energy Energy • Property of a system that enables it to do work. Work • So, the word work, in science terms, means something transferred energy somehow. • Units for work and energy in science terms are Joules, in human terms they are Calories or kWh Ch07 - Energy - Revised: 2/21/10

5. Derivation of W into E Ch07 - Energy - Revised: 2/21/10

6. Work involves force and distance (but really displacement). • Work = Force  displacement • W = F x d • J = N x m (units) • J = kg * m/s2x m = kg m2/s2 whenever useful mechanical work is done: • Something changes the energy associated with that thing Ch07 - Energy - Revised: 2/21/10

7. But there is a catch • The force must be in the same direction as the motion • Is this dude doing any work? • What if he was dragging the bag? Ch07 - Energy - Revised: 2/21/10

8. But there is a catch • You might be saying, “Grist you are nuts. If I carried a heavy bag a long way I would get tired. So, I did work!” • Nope! You may have exerted effort, but you did not change the energy of the bag. Ch07 - Energy - Revised: 2/21/10

9. Can you do negative work? • Yes, when F & d are in opposite directions. • Ex: setting down a bag of groceries. W = F cos ϑ d = F cos (180) d = F (-1) d = -Fd = -W Ch07 - Energy - Revised: 2/21/10

10. Work CHECK YOUR NEIGHBOR If you push against a stationary brick wall for several minutes, you might get tired, but did you do any work? A. yes. • no. • Both of the above. • None of the above. Ch07 - Energy - Revised: 2/21/10

11. An Example of Work A weightlifter raising a barbell from the floor does work on the barbell. Increasing it’s height increases its potential energy. If he dropped it on your foot it would do work on your foot. Does he do work while holding it over his head? Does he do work while holding it up and walking around? Does he do work if it’s on the ground and he drags it around? Ch07 - Energy - Revised: 2/21/10

12. Power Power: • Measure of how fast work is done • In equation form: Ch07 - Energy - Revised: 2/21/10

13. Units of Power Unit of power • WATT is the unit of power? • Named after James Watt, developer of the steam engine • Horsepower • Named after a horse that probably carried heavy stuff or pulled things, also eater of grass and maker of manure • 750 Watts = 1 horsepower Ch07 - Energy - Revised: 2/21/10

14. Power Examples • A worker uses more power running up the stairs than climbing the same stairs slowly. Tor F • If he were to run the 2 flights of stairs would he produce more power than running just one? Yes /No • If you drove your car twice as far as usual, would your car’s engine become more powerful? Yes /No • An engine with twice the power can do twice the work of one engine in the same amount of time. T/ F Ch07 - Energy - Revised: 2/21/10

15. 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. Ch07 - Energy - Revised: 2/21/10

16. Mechanical Energy Mechanical energy is due to position or to motion, or both. There are two forms of mechanical energy: • Potential energy – position • Kinetic energy - motion Ch07 - Energy - Revised: 2/21/10

17. Potential Energy Stored energy potential for doing work Example: • A stretched bow has stored energy that can do work on an arrow. • A stretched rubber band of a slingshot has stored energy and is capable of doing work. Ch07 - Energy - Revised: 2/21/10

18. Potential Energy—Gravitational Potential energy due to elevated position. Example: • water in an elevated reservoir • Cat at the top of a tall tree Ch07 - Energy - Revised: 2/21/10

19. Potential Energy—Gravitational • Equal to the work done in lifting it Ch07 - Energy - Revised: 2/21/10

20. Kinetic Energy • Energy of motion • Depends on the mass of the object and speed squared KE = ½mv2 • If object speed is doubled  kinetic energy is quadrupled. • If you double the speed you drive you quadruple you stopping distance. (slow down!!) Ch07 - Energy - Revised: 2/21/10

21. Law of Conservation of Energy Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. Ch07 - Energy - Revised: 2/21/10

22. Kinetic Energy and Momentum Compared Kinetic energy is not momentum!!!! Since we are not doing a unit on momentum, ask Grist or Dickson what the difference is if you are curious. Ch07 - Energy - Revised: 2/21/10

23. Energy for Life • Body is a machine, so it needs energy. • Average people need 2000 Calories per day. • This is equals 2.2 kWh about 25 cents worth of energy from LPEA Ch07 - Energy - Revised: 2/21/10

24. Cool energy fact Example: • Photovoltaic cells on rooftops catch the solar energy and convert it to electricity. More energy from the Sun hits Earth in 1 hour than all of the energy consumed by humans in an entire year! Ch07 - Energy - Revised: 2/21/10