Work and Simple Machines Chapter 14

1. Work and Simple Machines Chapter 14 • I can : • 1. Recognize when work is done • 2. Calculate how much work is done • 3. Explain the relation between work and • power.

2. What is work? In Science, Work is done when a force causes an object to move in the same direction that the force is applied.

3. In order for you to do work: Two things must occur: • You must apply force on an object. • (Force is a push or pull that changes the motion or shape of an object.) • 2.The object must move in the same direction as your applied force.

4. You do work on an object ONLY when the object moves as a result of the force you exerted. Work involves MOTION, not just effort.

5. The woman may think she is working by holding the bags. However, if she is not moving, she is NOT doing work because she is not causing something to move.

6. To do work, how must a force make an Object move?

7. In the direction of the force. When the boy walks forward NO WORK is being done by the upward force that his arms exert.

8. Work is done only when the FORCE you exert on an object is in the same direction as the object’s motion; Lifting a clothes basket is work, but carrying it while walking is not work.

9. The boys arms DO work when they exert an Upward force on the weights and the weights move Upward. Whenever you apply force to an object, the object moves in the direction of the force.

10. True or False: Applying a force ALWAYS results in work being done.

11. Force in two directions: Sometimes only part of the force you exert moves an object. Think about what happens when you push a lawn mower. You push at an angle to the ground. Only the part of the Force that is in the SAME direction as the motion of the mower does work.

12. Calculating Work Work is done when a force makes an object move. More work is done when the force is increased or the object is moved a greater distance. In SI units, the unit for work is the joule, named for James Prescott Joule Work Equation Work ( joules) = force ( newtons ) x distance( meters) W=Fd

13. Distance in the work equation is the distance an object moves only WHILE the force is applied.

14. Applying Math Work - A painter lifts a can of paint that weights 40 N a distance Of 2 m. How much work does she do? Hint: to lift a can weighing 40 N, the painter must exert a Force of 40 N. This is what you know: force: F = 40 N distance: d = 2m This is what you need to find out: work: W = ?J

15. This is the procedure you need to use: Substitute the known values F = 40 N and d = 2 m Into the work equation: W =Fd = (40 N) (2m) = 80 Nm = 80 J Check your answer by dividing the work you Calculated by the distance given in the problem. The result should be the force given in the problem.

16. Practice Problems: • As you push a lawn mower, the horizontal force is 300 N. If you push the mower a distance of 500 m, how much work do you do? • A librarian lifts a box of books that weighs 93 N a distance of 1.5 m. How much work does he do?

17. Answers: • Work = force x distance = 300 N x 500 m = 150,000J • Work = force x distance = 93N x 1.5 m = 140 J

18. What is Power? Imagine two weightlifters lifting the same amount of weight the same vertical distance. They both do the same amount of work. However, the amount of power they use depends on how long it took to do The work. Power is the rate at which work is done.(How quickly work Is done.) The weightlifter who lifted the weight in less time is more powerful.

19. In SI units, the unit of power is WATT, in honor Of James Watt, a nineteenth-century British Scientist who invented a practical version of the steam engine.

20. Calculating Power: Power can be calculated by dividing the amount of WORK DONE by the TIME NEEDED to do the work. Power Equation power = work (joules) time (seconds) P = W T

21. Applying Math Power - You do 200 J of work in 12 s. How much power Did you use? Work : W = 200J ; time: t = 12 s Power: P = ? Watts P=W =200 J = 17 watts T 12 s Check your answer by multiplying the power you Calculated by the time given in the problem. The results should be the work given in the problem.

22. Practice Problems • In the course of a short race, a car does 50,000 J of • work in 7 s . What is the power of the car during the race? • 2. A teacher does 140 J of work in 20 s. How much power did • Use?

23. Answers: • Power = work done/time needed • 50,000 J/7s = 7,143 watts • 2. P =W/t = 140 J/20s = 7 watts

24. What is Energy? The ability to do work. Example: I must have energy to run the mile.

25. Work and Energy If you push a chair and make it move, you do work on the chair and change its energy. Recall that when something is moving it has energy of motion, or kinetic energy. By making the chair move you increase its kinetic energy. You also change the energy of an object when you do work and lift it higher. An object has potential energy that increases when it is higher above Earth’s surface. By lifting an object, you do work and increase its potential energy.

26. Power and Energy: When you do work on an object you INCREASE the KINETIC energy of the object. Because energy can never be created or destroyed, if the object gains energy then you must lose energy. When you do work on an object you transfer energy to the object, and your energy decreases. The amount of work done is the amount of ENERGY TRANSFERRED in a certain amount of time. Sometimes energy is transferred even when no work is done, such as when heat flows from a warm to a cold object. In fact, there are many ways energy can be transferred even if no work is done. Power is always THE RATE at which energy is transferred- the amount of energy transferred divided by the time needed.

27. Is carrying a book while walking work? Why or why not?

28. Today we have learned: 1. Work is done when a force causes an object to move in the same direction that the force is applied. 2. Power is the rate at which work is done. 3. Something is more powerful if it can do a given amount of work in less time.

29. Review Questions: 1. When a force causes motion to occur in the Same direction in which the force has been applied, we say that _____________has been done. 2. Suppose you are waiting for a train. While You are standing on the platform, your arms are becoming more and more tired from holding heavy Suitcases. Are you doing work?

30. In SI, the unit for work is the ______________. • Ampere • Joule • Newton • Watt

31. I can: Explain how a machine makes work easier Calculate the mechanical advantages and efficiency of a machine. Explain how friction reduces efficiency.

32. What is a machine? • A machine is simply a device that makes doing work easier.

33. What is a Simple Machine? • A simple machine has few or no moving parts. • Simple machines make WORK EASIER. • Even a sloping surface can be a machine.

34. Mechanical Advantage • Machines do not decrease the amount of work you need to do. • A machine changes the WAY in which you do work. • You exert a force over some distance.

35. Input Force • The force you apply on a machine is the INPUT FORCE. • The work you do on the machine is equal to the input force times the distance over which your force moves the machine. • The work that you do on the machine is the input work.

36. Output Force • The machine also does work by exerting a force to move an object over some distance. • The resistance force. • When using a machine, the output work can never be GREATER than the input work.

37. When you use a machine, the output work can Never be greater than the input work.

38. Changing Force • Some machines make doing work easier by reducing the force you have to apply to do work. This type of machine increases the input force, so that the output force is greater than the input force.

39. Changing Force • The number of times a machine increases the input force is the mechanical advantage of the machine.

40. Mechanical Advantage • What is the advantage of using a machine? • A machine makes work easier by changing the amount of force you need to exert, the distance over the force exerted, or the direction in which you exert your force.

41. Mechanical Advantage • Number of times the input force is multiplied by a machine-Equation: • MA=F out/F in

42. Efficiency • The ability of a machine to convert input work to output work; calculated as : • efficiency=output work/input work x 100% • eff = W out x 100 W in

43. Friction • To help understand friction, imagine pushing a heavy box up a ramp. • As the box begins to move, the bottom surface of the box slides across the top of the ramp. • Neither surface is perfectly smooth-each has high spots and low spots.

44. Friction • Reduces efficiency by converting some work into HEAT. • The efficiency of a real machine is always LESS THAN 100 percent because of friction.

45. Friction and Efficiency • One way to reduce friction between two surfaces is to add oil. • Oil fills the gaps between surfaces and keeps many of the high spots from making contact. • More input work is converted to output work by machine.

47. Infer how the output work done by a machine changes when friction is reduced.

48. Discussion Question: • What is the efficiency of a machine and how can it be improved?

49. We have learned there are three main advantages to a machine. • By changing the amount of force you need to exert. • 2 .Changing the distance over which the force is • exerted • 3. Changing the direction in which you exert the force.

50. Exit slip: What is the force that you apply on a machine? The force that the machine applies is the_________.