Energy, Work & Power

1. Energy, Work & Power Sec. 3.5, 3.7, 3.9

2. Is he doing any WORK? He’s applying a FORCE, but is anything moving as a result of his applied force? FN FA Balance forces, no change in motion

3. Energy Doing work require ENERGY! Energy is the ability to do work Work is a transfer of energy

4. Work • Work = force x displacement • W = f x d • In every case in which work is done, work requires 2 things: • A force is applied to an object • The object moves a distancein the direction of the force W f d

5. Example • When a weightlifter lifts a barbell, he does workon the barbell • When he is holding the barbell over his head, he does no work (on the barbell)

6. Work or NO work • A lifeguard sitting on her stand _____ • Mowing the lawn. _____ • Pushing on a wall _____ • Carrying a box 10 meters. _____ • Writing out your homework _____ • Watching television _____ • Eating a burger. _____ • Running in place. _____

7. Units for Work • The unit of work combines the unit of FORCE • Newtons • With the unit of DISPLACEMENT / DISTANCE • Meters • It’s called a Newton-meter • But…Since work requires ENERGY, then use the units for energy: Joule (J) • 1 N of force to lift an 1 m = ____ Joule of work

8. Work = F x d How much WORK is done by applying 15 N of force to lift a chair 2 m above your head?

9. Example of Work How much work is done to a 10 kg mass that is lifted 2 m? Force weight = m x g Work w = f x d

10. Who’s doing more work? They are both doing the SAME work: Applying a FORCE on an object (dirt) to move it a distance.

11. Lawnmower Engine Jet Engine Which has more POWER? They burn the same fuel… 30 minutes of work …but at different rates 0.5 s of work

12. Power • Power is the RATE at which WORK is done • Power = work done time or P = force x distance time w p t

13. Hair dryer vs. Air drying your hair • What is the WORK that needs to be done?

14. Units for Power The unit for work is the Joule of energy The unit for time is seconds So...The unit for power is Joules/second (J/s) Also called the Watt. After James Watt

15. 1 J/s = 1 Watt 1000 W = kilowatt 1 million Watts = 1 mega- watt

16. practice • A parent is pushing a child on a bicycle down the street. If 125 J of work is done on a child’s bicycle for 120 seconds, what was the power output of the parent?

17. practice • Anna walks upstairs on her way to class. • She weighs 500 N and the stairs go up 3.25 m. • What is her power output if she climbs the stairs in 12.6 seconds?

18. Hair dryer 1875 what?

19. more powerful dryer hair

20. Power • Which are you more tired… • Walking a box up a flight of stairs or running a box up a flight of stairs? • You are doing the same amount of work, so why are you more tired? • POWER • Power is the measure of how fast work is done

22. Work and Power Lab Purpose: How much work is done, how much power is used to climb a flight of stairs. Prediction:

23. Work = f x d Weight = m x g (N) Power = work time Distance = 1st floor to 2nd floor (meters)

24. Work = f x d Weight = m x g (N) Power = work time Distance = 1st floor to 2nd floor (meters)

25. Conclusion Questions • Who did more work: walker or runner? • Explain what they did: • Who used more power? • Why?

26. Graph #1 Work done

27. Power graph Joules

28. Things to think about…. • If you push an object twice as far while applying the same force, you do • Four times as much work • The same amount of work • Twice as much work • ½ as much work • A job is done slowly, while an identical job is done quickly. Both jobs require the same amount of work, but different amounts of • Energy • Power • Work • None of these

29. Things to think about…. • Which requires more work: Lifting a 10 kg sack a height of 2 m, or lifting a 5 kg sack a height of 4 m? • The 10 kg sack • The 5 kg sack • They require the same amount of work • Whereas impulse is the time that a force acts, work involves the • Time and distance that a force acts • Distance that a force acts • Acceleration that a force produces

30. When James Watt invented his steam engine, he was asked how many horses the engine could replace. To find out, Watt built a rope and pulley device which he could measure the time it took a horse to lift various weights. • Watt defined horsepower as the power a horse can produce steadily for a period of time. • One horsepower is equal to 746 Watts.

31. Horsepower • A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute.

32. Just for the fun of it, figure out how many horsepower the runner in your group has…to do this is simple. • Divide the power in watts that you figured in your data chart by 746. That’s how much horsepower you have! HP = power in watts 746

33. Most hp cars? #1. 2009 Bugatti Veyron (1001 hp) #2. 2010 Lamborghini Murcielago (632 hp) #3. 2009 Mercedes-Benz SLR McLaren (617 hp) #4. 2009 Ferrari 599 GTB Fiorano (612 hp) 2009 Ford Escort (110 hp

34. Now you play! • Ramps Game! http://www.gamegecko.com/ramps.php • Parachute Game! www.bassfiles.net/parachute.swf

35. Poster Activity • Choose: Poster, Comic strip, short story… • Must include all of the following: • Momentum • Impulse • Force, time • Energy (KE and PE) • Work • Power