1. CH 8: Energy

2. Last chapter: How long a force acts makes a difference in an object’s motion. F x t = Impulse

3. This chapter: How long (how far) a force acts, which is known as work.

4. 1. Which takes more work lifting 1 book from the floor to your waist or lifting 10 books from the floor to your waist? Why?

5. 2. Which takes more work, lifting 1 book from the floor to your waist or lifting 1 book from the floor to over your head? Why?

6. 3. Which requires more work holding 100 pounds stationary over your head or holding 300 pounds stationary over your head? Why?

7. Work A. product of a force acting in the direction of motion and the distance moved. B. Work = force x distance W = F x d C. Units for work Joules (J) = N m

8. Practice problem: How much work is done on a 50 N skater that is lifted by her partner 1.5 m into the air?

9. Another Problem: Who does more work, a person who lifts 100 pounds 2 m over their head very quickly or a person who struggles and takes a long time to lift the 100 pounds 2 m over their head? Why?

10. Power A. rate at which work is done B. Power = work done time interval P = W/ t C. Units for power watts (w) = joules / t horsepower (hp) 1 hp = 747.7 w

11. Practice problem: How much power is required to lift Bertha, (who weighs 70 N) 20 m in an elevator in 6 s?

12. Energy A. ability to do work B. Units = joules C. Mechanical energy

13. 1. Potential Energy a. stored energy b. energy due to position c. examples rubber band spring fuel - gas, batteries, food a book held high

14. 1. Potential Energy d. potential energy is obtained by work being done. e. PE = mgh

15. 2. Kinetic energy a. energy of motion b. energy due to movement c. depends on mass and speed d. KE = 1/2 mv 2

16. Conservation of Energy Energy can not be created or destroyed, only transformed from one form to another. Total amount of energy never changes.

17. Conservation of Energy 3. Energy transforms from one form to another

18. Sunlight Heat Chemical Mechanical Electrical Plants Warm air rises Solar cells

19. IV. Machines A. device used to multiply forces or change the direction of forces B. basic idea is same as conservation of energy C. work in = work out

20. work in = work out (F. d) in = (F. d) out F. d = f. d 10 N x 1 m = 1 N x 10 m

21. D. lever F. d = f. d

22. E. Mechanical Advantage MA = output force input force pg. 11280 N = 8 10 N MA = input distance output distance

23. F. Pulley (bonus question on the test)

24. G. Efficiency 1. ideal machines have 100% efficiency Does not happen in the real world (some energy is ALWAYS lost as heat)

25. G. Efficiency Work (out) Eff = Work (in) X 100% OR useful work output X 100% total work input AMA Eff = TMA X 100%