Work, power and energy

1. Work, power and energy

2. Warm Up 5/2/12 • What is the first law of motion? • Which formula is associated with the 2ndlaw of motion? • What is the first thing to come to your mind when you hear the word work, power and energy?

3. Announcements • You will have a unit test on Motion and Forces next Friday  • If you have any make up work you need to get it to me by Friday, May 4th if you want it reflected on your upcoming progress report.

4. What is work? • Work is the use of force to move an object. • In scientific terms, you only do work when you use force to move something.

5. How do we calculate work? Work= force X distance OR W= FD The unit that we use to measure work is Joule (J) or the Newton-meter

6. Example 1 • How much work is done if a person lifts a barbell weighing 500 N to a height of 2 m?

7. Answer Formula for work is W=FD W=? F=500 N D=2 m W= 500/2 W=250 J

8. Other formulas If you have to calculate distance: • D= W/F • The unit is meters If you have to calculate force: • F= W/d • The unit is Newton’s

9. Guided Practice Problems • If you push a cart with force of 70 N for 2 M, how much work is done? • If you did 200 J of work pushing a box with a force of 40 N, how far did you push the box? • If you get a flat tire and have to push a car 3 M and apply a force of 3 N, how much work did you do? • How much force is needed to move a tractor trailer if a machine is only allowed to use 4 J of work and it has to move 25 M.

10. Who or what does work? • Just as you do work when you pick up a book or write in your notebook, objects can also do work. • For example, cars do work when the move down the road, or bowling balls do work when they hit pins.

11. Independent Practice • If you push very hard on an object but it does not move, have you done work? Explain why or why not. • What two factors do you need to know to calculate how much work was done in any situation? • Was work done on a book that fell from a desk to the floor? If so what force was involved? • Work is done on a ball when a soccer player kicks it. is the player still doing work on the ball as it is rolls across the ground? • Tina lifted a box 0.5 M. the box weighed 25 N. how much work did Tina do?

12. Answers • No, the object must move for work to be done. • Force and distance. • Yes, the force of gravity. • No: the player is no longer exerting force on the ball. • W= FD = 25 N x 0.5 m = 12.5 J

13. Warm Up 5/3/12 • What is the formula for work? • What unit do we use for work? • Can objects do work or just people? Be sure to explain your answer.

14. Announcements • Any missing work that you want reflected on the upcoming progress report must be given to me by tomorrow. • Do not hand it to me or place it on my desk….on your way out of my room, please place it in the late work bin.

15. Power • Power is the rate at which work is done. • Power = Work/ Time • The unit of power is the watt.

16. Check for Understanding 1.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? Explain your answers.

17. 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. Power and time are inversely proportional.

18. 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts

19. 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time

20. Example 1 • An explorer uses 6000 J of work to pull his sled for 60 seconds. What power does he need?

21. Answer P= w/t P= 6000 j/60 s P= 100 W

22. Practice Problems • If a conveyor belt uses 10 J to move a piece of candy a distance of 3 m in 20 s, what is the conveyor belt’s power? • An elevator uses a force of 1710 N to lift 3 people up 1 floor. Each floor is 4 m high. The elevator takes 8 s to lift the 3 people up 2 floors. What is the elevators power?

23. Solutions to practice problems • P= w/t P= 10 J/20 s = 0.5 j/s P= 0.5 W • P= w/t W= fd P=1710 N x 8 m 8 s P= 1710 W

24. History of Work Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

25. Does this sound familiar? • If you think about cars, they use the unit horsepower. • Horsepower is the amount of work a horse can do in a minute. • We still use this unit as long ago, many people used horses to do work.

26. Independent Practice • How is power related to work? • What do you need to know to calculate how much energy a light bulb uses? • Which takes more power: using 15 N to lift a ball in 5 seconds or using 100 N to push a box 2 m in 1 minute?

27. Answers • Power is the rate at which work is done. The faster you do work, the greater your power. • Power can be measured in joules per second (watts) or in horsepower. Examples will vary but might include light bulbs for watts and cars for horsepower. • Using 15 N to lift a ball 2 m in 5 seconds.

28. Warm Up 5/4/12 • What is power? • How do you calculate power? • What unit do we use to calculate power? • Why is it useful to us in science class?

29. Announcements • All make up work due today. • Please put it in the late work bin on your way out today  • Check the no name folder to see if you have work there. It will be thrown away today @ 4:15.

30. Energy • This is the ability to do work. • Work transfers energy. There are three different types of energy: • Potential energy • Kinetic energy • Mechanical energy

31. Kinetic energy • This is energy in motion. • For example, when you throw a ball you transfer energy and it moves. • By doing work on the ball (throwing it), you give it kinetic energy. • Formula: KE= 1/2mv2

32. Potential energy • This is stored energy or the energy that an object has due to its position or shape. • For example, when you do work to lift a ball from the ground you give the ball potential energy. Why? Because it has the “potential” to be thrown or fall back to the ground.

33. Mechanical energy • Is the energy possessed by an object due to its position or motion. Or ME= PE + KE

34. Example of KE • What is the kinetic energy of a girl who as a mass of 40 kg and a velocity of 3 m/s?

35. Answer KE= ½ mv2 = ½ x 40 kg x (3 m/s)2 =360kg 2s =180 J

36. Other types of energy • Thermal energy • Chemical energy • Nuclear energy • Electromagnetic energy

37. Stations

38. Independent Practice • Which type of energy do you think is best. Be sure to explain why.

39. Warm Up 5/7/12 • Explain the relationship between work and energy. • What is the formula for kinetic energy? • What is the formula for mechanical energy? • Briefly explain the difference between potential and kinetic energy.

40. Announcements • Be sure to turn in homework 

41. Simple Machines Ancient people invented simple machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

42. Simple Machines • The six simple machines are: • Lever • Wheel and Axle • Pulley • Inclined Plane • Wedge • Screw

43. Simple Machines • A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: • transferring a force from one place to another, • changing the direction of a force, • increasing the magnitude of a force, or • increasing the distance or speed of a force.

44. Mechanical Advantage • It is useful to think about a machine in terms of the input force (the force you apply) and the outputforce (force which is applied to the task). • When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

45. Mechanical Advantage • Mechanical advantage is the ratio of output force divided by input force. If the output force is bigger than the input force, a machine has a mechanical advantage greater than one. • If a machine increases an input force of 10 pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10. • In machines that increase distance instead of force, the MA is the ratio of the output distance and input distance. • MA = output/input

46. No machine can increase both the magnitude and the distance of a force at the same time.

47. The Lever • A lever is a rigid bar that rotates around a fixed point called the fulcrum. • The bar may be either straight or curved. • In use, a lever has both an effort (or applied) force and a load (resistant force).

48. The 3 Classes of Levers • The class of a lever is determined by the location of the effort force and the load relative to the fulcrum.

50. To find the MA of a lever, divide the output force by the input force, or divide the length of the resistance arm by the length of the effort arm.