Understanding Work, Power, and Efficiency in Science

1. Objectives: • Learning Intentions – • Students will be able to: • Describe work in the scientific mean • Solve for work when given force and distance • Success Criteria – • We will know we are successful when we can accurately determine if work is done in a scenario • Accurately solve for work when given the force and distance

2. Think about it… • If you are sitting at home pushing buttons on the remote control, how much work are you doing? • In which instance am I doing more work: • A) I carry 10 books across the room all at once • B) I carry the 10 books across the room one at a time

3. What it Means to Do Work

4. The word work is used in many ways: • You work on science problems • You go to work • Your toaster doesn’t work • Taking out the trash is too much work

5. Amount of force applied to an object over a distance. In order for you to do work the object must MOVE in the same direction as the force Work

6. What’s work? • A scientist delivers a speech. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?

7. What’s work? • A scientist delivers a speech. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 km grocery bag to her car? No

8. W = work done by the force F = force on object along the object’s line of motion D = distance object moves while force is acting Work = Force X Distance

9. Units for Work newtonXmeter = joule (J)

10. Work is done only while the force is applied.

11. Effects of Work • Change the position of an object. • Change the velocity of an object.

12. F = 500 N d = 20 m W = Force X Distance EXAMPLE: W = 500 N X 20 m = 10,000 J How much work does it take to move a 500 N car a distance of 20 m?

13. Independent Practice: • Complete the back side of the work sheet on your own or with a partner • Use your notes from class • If you have questions raise your hand • We will review some of the answers before the end of class

14. Do Now: Thursday, Jan. 30 • In each of the following situations determine whether or not work was done: • An ice skater’s partner lifts her up a distance of 1 m. • The ice skater’s partner carries her across the ice a distance of 3 m. • After practice, the skater lifts her 20-N gym bag up 0.5m. • You pull your sled through the snow a distance of 500 m with a force of 200 N. How much work did you do?

15. Objectives: • Learning Intentions – • Students will be able to: • Describe power in a scientific way • Solve for power given work and time • Success Criteria – • We will know we are successful when we can accurately determine the power of a scenario given work and time

16. POWER

17. POWER IS THE RATE AT WHICH WORK IS DONE OR THE AMOUNT OF WORK PER UNIT OF TIME.

18. Power = Work time ÷

19. UNITS Force = N Distance = m Time = sec Work = J Power = J/s = watts = W

20. Units for Power 1 joule per second = 1 watt (W)

21. Power • Given: A motor does 6000 J of work in 40 seconds. • Determine the power of the motor in watts.

23. How much power does a person weighing 750 newtons need if he takes 20 seconds to walk up a staircase 10 meters high?

24. Check for Understanding Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power?

25. Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time.

26. Review of Equations: • Work = force X distance • Force = work ÷ distance • Distance = work ÷ force • Power = work ÷ time • Work = power X time • Time = work ÷ power

27. Independent Practice: • Complete the practice problems for work and power • Any questions let me know • Put the completed sheet in your class tray. If you do not complete it DO SO FOR HOMEWORK • We will review some problems before the end of class

28. Do Now: Fri. Jan. 31 Fill in the chart.

29. Objectives: • Learning Intentions – • Students will be able to: • Explain the concept of efficiency • Solve for efficiency of a machine or scenario • Success Criteria – • We will know we are successful when we can accurately solve for the efficiency of a machine

30. Review of Equations: • Work = force X distance • Force = work ÷ distance • Distance = work ÷ force • Power = work ÷ time • Work = power X time • Time = work ÷ power

31. Recall: Work • Work is the amount of force applied to an object over a distance. • Work = Force X Distance • Unit: Joule

32. Efficiency and Power • Every process that is done by machines can be simplified in terms of work: • work input: the work or energy supplied to the process (or machine). • work output: the work or energy that comes out of the process (or machine).

33. Efficiency • The efficiency of a machine is the ratio of usable output work divided by total input work. • Efficiency is usually expressed in percent. Output work (J) efficiency = Wo Wi x 100% Input work (J)

34. Independent Practice: • Complete the following practice problems on work, power and efficiency • Questions, let me know • Should be completed before the activity on Monday