Work, Power, and Simple Machines

1. Work, Power, and Simple Machines

2. What is Work? • Work is done when a force is exerted on an object and causes it to move in the same direction the force was exerted. • Force – a push or pull

3. The amount of work you do is not affected by how fast or slow you do the work. If you carry a backpack up a flight of stairs, you are still carrying the same amount of weight over the same amount of distance. Time is not a factor in work.

4. Calculate Work • Work (W) = Force(F) x Distance(D) • W = F x D • Force is measured in Newtons (N) • Distance is measured in meters (m) • So, Work is measured in N*m or Joules (J)

5. What is a Joule? • A joule is the amount of work you do when you exert a force of 1 Newton to move an object a distance of 1 meter.

6. W = ? W = F x D F = 50N D = 0.5m If a 50N force was exerted lifting a plant and carrying it a distance of 0.5m, how much work was done?

7. W = F x D • W = (50N) (0.5m) • W = 25 N*m • OR • 25 Joules (J)

8. What is the SI unit for work? • The Joule (J)

9. What is Power? • Power is the RATE at which work is done. • Rate = Time

10. Calculating Power • Power(P) = Work(W) / time(t) • P = W/t • P = Force x Distance / time • Work is measured in Joules (J) • Time is measured in seconds (s) • Power is measure in J/s • J/s = Watts

11. A tow truck exerts a force of 11,000N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? • P = ? • P = F x D / t OR P = W/t • F = 11,000 N • D = 5 m • t = 25 s

12. P = 11,000 N x 5 m / 25 sP = 55,000Nxm(J) / 25 sP = 2,200 J/s OR watts

13. A watt is a relatively small unit of power. • One kilowatt (kW) = 1,000 watts (W) • One horsepower = 746 watts

14. How Machines Do Work? What is a Machine? A machine allows you to do work easier or more effective. HOW?

15. 3 ways in which a machine can make work easier. • A machine may change the amount of force you exert. By taking some of the force away from you. • The distance over which you exert your force. • Or their direction in which you exert your force.

16. Remember that in order for work to be done a force must be applied. You have an input force and an output force

17. Input force/Output force • The force you exert on the machine – input force. • The machine in turn applies the force to the object – Output force • The distance the machine moves after the input force is applied - input distance • Output distance – the machine exerts a force over another distance, distance object moves

18. Changing force

19. Increases Distance

20. Changes direction

21. Mechanical Advantage • The number of times a machine increases a force exerted on it. • Mechanical advantage = output force/ input force • MA = Fo / Fi

22. Calculating Mechanical Advantage • What is the MA if the input force of 10N is exerted on a hand-held can opener, and the opener exerts an output force of 30N? • MA = ? • MA = output force / input force • Output force = 30N • Input force = 10N

23. MA = 30N / 10N • MA = 3 • The can opener triples your input force.

24. Are machines actually efficient? • Efficiency – compares the output work to the input work. Efficiency is expressed as a percent. The higher the percent the more efficient the machine.

25. Calculating Efficiency • Efficiency = Output Work / Input Work x 100% • Eff = Wo / Wi Multiplied by 100% • We multiply because we end up with a decimal, so we move out decimal two places to the right.

26. Example efficiency • You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? • Efficiency = ? • Eff = output work / input work x 100% • Work input = 250,000 J • Work output = 200,000 J

27. Eff = 200,000J / 250,000J x 100%.8 x 100%eff = 80%

28. What would your ideal efficiency of a machine be? • 100% efficient

29. Inclined plane Screw lever Wedge Pulley Wheel and axle Simple Machines

30. What is a simple machine? • Remember a machine makes work easier by changing the force, the direction, or the distance the force is applies.

31. 1. Inclined Plane • A flat, sloped surface • Allows you to exert your input force over a longer distance. • As a result the input force needed is less than the output force • Ideal MA = Length of incline / height of incline

33. 2. Wedge • A device that is thick at one end and tapers to a thin edge at the other end. • Axe • Knife • Door stop

35. 3. Screw • An inclined plane wrapped around a cylinder. • The MA of a screw is the length around the threads divided by the length of the screw.

36. 4. Levers • A rigid bar that is free to pivot, or rotate on a fixed point. • MA = distance from fulcrum to input forcedistance from fulcrum to output force

37. Types of Levers • 1st class lever • 2nd class lever • 3rd class lever

38. 1st class lever

39. 2nd class lever

40. 3rd class lever

41. Wheel and Axle • Made of two circular or cylindrical objects fastened together that rotate about a common axis • Screw driver

42. 6. Pulleys • Made of a grooved wheel with a rope or cable wrapped around it.

43. Types of Pulleys • Fixed pulley • Movable pulley • Block and tackle

44. Fixed pulley

45. Movable pulley

46. Block and Tackle

47. 1.  A fork is an example ofa: pulley       wedge       wheel and axle   2.  A bicycle is an example of a:  wheel and axle       wedge       lever  3.  A bathtub is an example of a/an:  pulley       screw       inclined plane  4.  A swivel chair is an example of a:  lever       screw       wedge  5.  A teeter totter on the playground is a:  lever       inclined plane       pulley  6.  You would use a pulley to:  cut food       hold pieces of wood together  help open and close venetian blinds

48. Simple Machines and the Human Body • Elbow • Neck • Ankle • Wedges/mouth

49. Compound Machines • A machine that utilizes two or more simple machines.

50. Homework • You are to make a foldable • List all 6 simple machines • For each machine show a picture and give its definition • You will need 4 halves sheets of paper