1. Work and Machines Chap 5

2. Work 5.1

3. What is Work?  Work transfer of energy that occurs when a force makes an object move W = Fd W:work (J) F:force (N) d:distance (m) 1 J = 1 N·m Distance must be in direction of force! F W d

4. Doing Work  If you push on your desk and nothing moves, have you done work? No  Conditions for work to be done: Force must make object move The movement must be in the same direction as the applied force.

5. Which is Work?  Lifting books?  Walking with books?

6. Work and Energy  When work is done, a transfer of energy always occurs.  If you carry a heavy box up a flight of stairs, you transfer energy form your moving muscles to the box (increasing its PE by increasing its height).

7. Lifting Problem Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W = ? WORK: W = F·d W = (30 N)(1.5 m) W = 45 J F W d

8. Dancing is Work A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W = ? WORK: W = F·dF = m·a F =(40kg)(9.8m/s 2 )=392 N W = (392 N)(1.4 m) W = 549 J during lift No work after lift. “d” is not in the direction of the force. F W d

9. Power  Power rate at which work is done measured in watts (W) P:power (W) W:work (J) t:time (s)

10. Power in Skating  A figure skater lifts his partner, who weighs 450 N, 1.5 m in 3.0 s. How much power is required? GIVEN: F = 450 N d = 1.5 m t = 3.0 s WORK : P = W ÷ t W = F·d W = (450 N)(1.5 m) = 675 J P = 675 J ÷ 3.0 s P = 225 W P W t

11. Videos  http://www.brainpop.com/science /motionsforcesandtime/work/ http://www.brainpop.com/science /motionsforcesandtime/work/

12. Self Check 5.1 1.The amount of work done depends on what two things? 2.You push a box with a force of 100 N. If it moves 5 m while you are pushing, how much work was done?

13. Using Machines 5.2

14. What is a Machine?  Machine device that makes work easier changes the size and/or direction of the exerted force

15. Making Work Easier  Machines make work easier by: increasing the force that can be applied to an object n car jack increasing the distance over which a force can be applied n ramp changing the direction of an applied force wedge-shaped blade of ax

16. Input and Output Forces  Input Force (F in ) force applied to the machine “what you do”  Output Force (F out ) force applied by the machine “what the machine does”

17. Force and Work  Work Input (W in ) work done on a machine  Work Output (W out ) work done by a machine W in = F in × d in W out = F out × d out

18. Conserving Energy  Conservation of Energy can never get more work out than you put in trade-off between force and distance W in = W out F in × d in = F out × d out

19. Ideal Machines  In an ideal machine...  But in the real world… some energy is lost as friction W in = W out W in > W out

20. Mechanical Advantage  Mechanical Advantage (MA) number of times a machine increases the input force MA > 1 : force is increased MA < 1 : distance is increased MA = 1 : only direction is changed

21. Open a Window  A worker applies an input force of 20 N to open a window with a output force of 500 N. What is the crowbar’s MA? GIVEN: F in = 20 N F out = 500 N MA = ? WORK : MA = F out ÷ F in MA = (500 N) ÷ (20 N) MA = 25 MA F out F in

22. Mechanical Advantage  Find the input force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: F in = ? F out = 2000 N MA = 10 WORK : F in = F out ÷ MA F in = (2000 N) ÷ (10) F in = 200 N MA F out F in

23. Mechanical Advantage  Window blinds change direction of the input force. The input and output forces are equal, so the MA = 1

24. Efficiency  Efficiency measure of how completely work input is converted to work output In real machines, it is always less than 100% due to friction

25. Ramps  A worker exerts a force of 500 N to push a 1500 N sofa 4.0 m along a ramp that is 1.0 m high. What is the ramp’s efficiency? GIVEN: F in = 500 N d in = 4.0 m F out = 1500 N d out = 1.0 m WORK : W in = (500N)(4.0m) = 2000 J W out = (1500N)(1.0m) = 1500 J E = 1500 J × 100% 2000 J E = 75% 1.0m 1500N 4.0m 500N

26. Increasing Efficiency  Machines can be made more efficient by reducing friction. A lubricant fills in the gaps between surfaces, enabling them to slide past each other more easily.

27. Videos

28. Self Check 5.2  Why is the work output always less than the input in a real machine?  Calculate the mechanical advantage of a hammer if the input force is 125 N and the output force is 2000 N.

29. Simple Machines 5.3

30. Simple Machines  A simple machine is a machine that does work with only one movement of the machine.  There are six types of simple machines: levers, pulleys, wheel and axle, inclined planes, screws, and wedges.

31. Levers  Lever a bar that is free to pivot or turn around a fixed point The fixed point the lever pivots on is called a fulcrum

32. Types of Levers  First class lever fulcrum in middle  Second class lever F out in middle  Third class lever F in in middle

33. IMA of a Lever  Ideal Mechanical Advantage length of input arm divided by length of output arm

34. Video  http://www.enchantedlearning.co m/physics/machines/Levers.shtm l http://www.enchantedlearning.co m/physics/machines/Levers.shtm l

35. Pulleys  Pulley grooved wheel with a rope or chain running along the groove a “flexible first-class lever” axel of pulley is the fulcrum L in L out F

36. IMA of a Pulley  Ideal Mechanical Advantage (IMA) equal to the number of supporting ropes IMA = 0IMA = 1IMA = 2

37. Fixed Pulleys  Fixed Pulley does not increase force changes direction of force IMA = 1

38. Movable Pulleys  Movable Pulley increases force doesn’t change direction IMA = 2

39. Comparing Pulleys  For a fixed pulley, the distance you pull the rope downward equals the distance the weight moves upward.  For a movable pulley, the distance you pull the rope upward is twice the distance the weight moves upward.

40. Block and Tackle Pulleys  Block & Tackle combination of fixed & movable pulleys may or may not change direction IMA = 4

41. Wheel and Axle  Wheel and Axle two wheels of different sizes that rotate together a pair of “rotating levers” Wheel Axle

42. Wheel and Axle  Is this skateboard a good example of a wheel and axel machine? No because the axle is fixed.

43. IMA Wheel and Axle  Ideal Mechanical Advantage (IMA) effort force is applied applied to at rim of wheel axle moves less distance but with greater force effort radius resistance radius

44. Inclined Planes  Inclined Plane sloping surface used to raise objects h l

45. Screw  Screw inclined plane wrapped in a spiral around a cylinder input force by turning screw output force on threads

46. Wedge  Wedge a moving inclined plane with 1 or 2 sloping sides changes direction of the input force

47. Compound Machines  Compound Machine combination of 2 or more simple machines

48. A. Compound Machines  Rube Goldberg Machine Rube Goldberg walks in his sleep, strolls through a cactus field in his bare feet, and screams out an idea for self-operating napkin: As you raise spoon of soup (A) to your mouth it pulls string (B), thereby jerking ladle (C) which throws cracker (D) past parrot (E). Parrot jumps after cracker and perch (F) tilts, upsetting seeds (G) into pail (H). Extra weight in pail pulls cord (I), which opens and lights automatic cigar lighter (J), setting off sky-rocket (K) which causes sickle (L) to cut string (M) and allow pendulum with attached napkin to swing back and forth thereby wiping off your chin. After the meal, substitute a harmonica for the napkin and you'll be able to entertain the guests with a little music.

49. Videos  http://www.brainpop.com/technology/simplemachines/levers / http://www.brainpop.com/technology/simplemachines/levers /  http://www.brainpop.com/technology/simplemachines/pulley / http://www.brainpop.com/technology/simplemachines/pulley /  http://www.brainpop.com/technology/simplemachines/wheel andaxle/ http://www.brainpop.com/technology/simplemachines/wheel andaxle/  http://www.brainpop.com/technology/simplemachines/inclin edplane/ http://www.brainpop.com/technology/simplemachines/inclin edplane/  http://computerkiddoswiki.pbworks.com/Simple-Machines http://computerkiddoswiki.pbworks.com/Simple-Machines  http://www.edheads.org/activities/odd_machine/ http://www.edheads.org/activities/odd_machine/

50. Self Check 5.3 1.Identify the simple machines in this bicycle.

52. Problems  You use a 160 cm plank to lift a large rock. If the rock is 20 cm from the fulcrum, what is the plank’s IMA? GIVEN: L r = 20 cm L e = 140 cm IMA = ? WORK : IMA = L e ÷ L r IMA = (140 cm) ÷ (20 cm) IMA = 7 IMA LeLe LrLr 20cm 160cm

53. Problems  A crank on a pasta maker has a radius of 20 cm. The turning shaft has a radius of 5 cm. What is the IMA of this wheel and axle? GIVEN: r e = 20 cm r r = 5 cm IMA = ? WORK : IMA = r e ÷ r r IMA = (20 cm) ÷ (5 cm) IMA = 4 IMA rere r 5 cm 20 cm

54. Problems  A steering wheel requires a mechanical advantage of 6. What radius does the wheel need to have if the steering column has a radius of 4 cm? GIVEN: IMA = 6 r e = ? r r = 4 cm WORK : r e = IMA · r r r e = (6)(4 cm) r e = 24 cm IMA rere r r rere

55. Problems  You need to lift a 150 N box using only 15 N of force. How long does the lever need to be if the resistance arm is 0.3m? GIVEN: F r = 150 N F e = 15 N L r = 0.3 m L e = ? MA = 10 WORK : L e = IMA · L r L e = (10)(0.3) L e = 3 m Total length = L e + L r Total length = 3.3 m IMA LeLe LrLr 0.3m ? 150N 15N

56. Problems  How much force must be exerted to push a 450 N box up a ramp that is 3 m long and 1.2 m high? GIVEN: F e = ? F r = 450 N l = 3 m h = 1.2 m WORK : IMA = l ÷ h IMA = (3 m)÷(1.2 m) IMA = 2.5 IMA l h MA FrFr FeFe F e = F r ÷ MA F e = (450 N)÷(2.5) F e = 180 N

57. Wedge  Zipper 2 lower wedges push teeth together 1 upper wedge pushes teeth apart