Chapter 3 – Work and Machines

1. Chapter 3 – Work and Machines 3.1 Work and Power

2. I. Essential Question: • What is work? • What is power?

3. II. Key Vocabulary • Work: any time you exert a force on an object that causes the object to move some distance. • Joule: the unit used to measure work. • Power: the rate at which work is done. • Watt:the unit used to measure power.

4. III. Key Concepts • If you push against a tree or car and it does not move, you are scientifically not doing work. • To do work, the object MUST move. • Likewise, if you carry an object like a backpack, you are not scientifically doing work because the motion is horizontal and the force is upward.

5. III. Key Concepts • To find work, multiply the force being used by the distance the object moves. • w = F x d • Measured in joules (J).

6. III. Key Concepts • An object that has more power than another object can do the same amount of work in a shorter amount of time. • To find power, multiply the force and distance together and then divide by the time. • Measured in watts (W); but usually measured in kilowatts (kW). • 1 horsepower = 746 watts.

7. Chapter 3 – Work and Machines 3.2 Understanding Machines

8. I. Essential Question: • What does a machine do?

9. II. Key Vocabulary • Machine: devices that allow you to do work in an easier way. • Input force: the force exerted when work is done. • Output force: the force exerted by the machine.

10. III. Key Concepts • Machines do not reduce the work done, they just change the way it is done to make it easier. • The input force is exerted over the input distance; the output force is exerted over the output distance. • When combined, the force and the distance equals the work for both input and output.

11. III. Key Concepts • Machines change work in three ways: • Changing force – the amount of work stays the same, so if you decrease the force needed to move an object, then that means you must move it for a longer distance. • Changing distance – the amount of work stays the same, so if you decrease the distance, then you must increase the force needed to move the object. • Changing direction – in some cases, the force and the distance remain the same. But, the motion the machine allows for is easier to do.

12. IV. Exploration

13. V. Understanding and Applying

14. Chapter 3 – Work and Machines 3.2 Understanding Machines

15. I. Essential Question: • What is mechanical advantage? • What is efficiency?

16. II. Key Vocabulary • Mechanical advantage: the number of times a machine increases a force exerted on it. • Efficiency: a comparison of output work to input work; expressed as a percentage.

17. III. Key Concepts • The ratio of output force to input force is the mechanical advantage. • When the output force is greater than the input force, the mechanical advantage is greater than 1. • When a machine increases distance, the output force is less than the input force, and the mechanical advantage is less than 1.

18. III. Key Concepts • When you change direction, the mechanical input is 1 – the input and output forces are equal. • All machines waste some work to overcome friction. • Using wax and other lubricants can reduce friction in machines, which makes them work with more efficiency.

19. III. Key Concepts • To find efficiency, divide the output work by the input work and multiply by 100%. • All machines have an efficiency less than 100%. • 100% would be ideal; less than 100% is actual.

20. IV. Exploration

21. V. Understanding and Applying

22. Chapter 3 – Work and Machines 3.3 Inclined Planes and Levers

23. I. Essential Question: • How do inclined planes work?

24. II. Key Vocabulary • Simple machine: the most basic device for making work easier. • Inclined plane: a flat, sloped surface. • Wedge: a device that is thick at one end and thin at the other. • Screw: an inclined plane wrapped around a cylinder.

25. III. Key Concepts • An inclined plane allows you to exert your force over a longer distance, like a ramp. • Input force = force you push or pull with; output force = object’s weight; input < output • To find the ideal mechanical advantage, divide the length of the incline by its height.

26. III. Key Concepts • A wedge is like two inclined planes put back to back. • Instead of moving an object on the inclined plane, you use the wedge itself. • Consider an ax. • Mechanical advantage: divide the length of the wedge by its width. • The longer and thinner the wedge, the greater the MA.

27. III. Key Concepts • A screw acts like an inclined plane by increasing the distance over which you exert the input force. • Bolts, drills, and jar lids are other examples. • Mechanical advantage: the length around the threads divided by the length of the screw. • The closer together the threads, the greater the MA.

28. IV. Exploration • Question: How does the height and length of a ramp affect how much work is needed to pull a weight? • Materials: Each group will need the following: • Meter stick • Wooden board (ramp) • Spring scale • String • 1 weight • 1 wooden block • Books to change the ramp height

29. IV. Exploration • Procedure: • 1. Write a hypothesis – “if…then…because…”. • 2. Make a low ramp using one or two books and the wooden board. • 3. Measure the length of the ramp and the height of the ramp. Measure to the nearest tenth of a centimeter. Record this information in your chart. • 4. Find the force (N) of the weight and the block together using the spring scale. Measure to the nearest tenth of a Newton. Record this information in your chart (under output force). • 5. Assemble a “sled” from your block and weight so that the weight will be able to travel up the inclined plane. Attach both of these to the spring scale, and begin pulling slowly.

30. IV. Exploration • Procedure: • 6. Once the “sled” is moving at a steady and slow pace, record the force being used by looking at the “N” on the spring scale. This should go in your “input force”. • 7. Make a higher ramp using more books. Repeat steps 3-6 with the higher ramp. • 8. To find the ideal mechanical advantage, divide the ramp length by the ramp height. • 9. To find the actual mechanical advantage, divide the output force by the input force. • 10. Answer the questions in your notebook.

31. IV. Exploration • Data Analysis:

32. V. Understanding and Applying • 1. How did your actual mechanical advantage compare to your ideal mechanical advantage? Give one reason to explain why you think you had these results. • 2. How does ramp height and ramp length affect work? • 3. What does your mechanical advantage tell you about your simple machine?

33. Chapter 3 – Work and Machines 3.3 Inclined Planes and Levers

34. I. Essential Question: • What are levers?

35. II. Key Vocabulary • Lever: a rigid bar that is free to pivot, or rotate, on a fixed point. • Fulcrum: the fixed point that the lever pivots around.

36. III. Key Concepts • A lever increases your input distance, which means you must apply a greater force to do the work. • Mechanical advantage: the distance from the fulcrum to the input force divided by the distance from the fulcrum to the output force. • Levers are classified based on the position of the fulcrum.

37. III. Key Concepts • First-class levers: the fulcrum is between the input and output. • Change direction of input force; increase force or distance • Seesaws and scissors • Second-class levers: the output force is between the input force and the fulcrum. • Increase force • Doors, nutcrackers, and bottle openers • Third-class levers: the input force is between the output force and the fulcrum. • Increase distance • Spoons, shovels, and baseball bats.

38. IV. Exploration

39. V. Understanding and Applying

40. Chapter 3 – Work and Machines 3.4 Putting Machines Together

41. I. Essential Question: • What simple machines use turning? • What is a compound machine?

42. II. Key Vocabulary • Pulley: simple machine made of a grooved wheel with a rope or cable wrapped around it. • Wheel and axle: made of two connected objects that rotate about a common axis. • Compound machine: two or more simple machines combined together.

43. III. Key Concepts • An inclined plane allows you to exert your force over a longer distance, like a ramp. • Input force = force you push or pull with; output force = object’s weight; input < output • To find mechanical advantage, divide the length of the incline by its height.

44. III. Key Concepts • A wedge is like two inclined planes put back to back. • Instead of moving an object on the inclined plane, you use the wedge itself. • Consider an ax. • Mechanical advantage: divide the length of the wedge by its width. • The longer and thinner the wedge, the greater the MA.

45. III. Key Concepts • A screw acts like an inclined plane by increasing the distance over which you exert the input force. • Bolts, drills, and jar lids are other examples. • Mechanical advantage: the length around the threads divided by the length of the screw. • The closer together the threads, the greater the MA.

46. IV. Exploration

47. V. Understanding and Applying