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Work, Power and Machines

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Work, Power and Machines

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  1. Work, Power and Machines

  2. Warm Up • How would you define work and energy? Do these words have the same meaning in science and in everyday speech? • What different types of energy do you know about?

  3. PSc. 3.1.3 • Explain scenarios in which work is done, identifying the force, displacement, and energy transfer. • Compare scenarios in which work is done and conceptually explain the differences in magnitude of work done using the relationship W=f x d

  4. THERMAL The ability to cause change. MECHANICAL NUCLEAR ELECTRICAL CHEMICAL joules (J) Energy internal motion of particles ENERGY motion of objects changes in the nucleus motion of electric charges bonding of atoms

  5. Which has the most KE? • Which has the least KE? 80 km/h 50 km/h 80 km/h Energy • Kinetic Energy (KE) • energy in the form of motion • depends on mass and velocity 80 km/h truck 50 km/h motorcycle

  6. Energy • Potential Energy (PE) • stored energy • depends on position or configuration of an object • Which boulder has greater gravitational PE? • What other ways can an object store energy?

  7. Work It’s not what you think! Work is a transfer of energy to a body by the application of a force that causes the body to move in the direction of the force. Huh? Work is using a force to change the position of an object.

  8. Work Work is Force times distance where Force is the force acting on the object (could be applied force, could be gravitational force) and distance is how far the object moved. W = F•d Units of work are Joules (J).

  9. Examples • A crane operator uses an average force of 5200N to lift a girder 25m. How much work does the crane do on the girder? • Given: F = 5200N • d = 25m • W = ? • W = F•d • W = 5200N x 25m W = 130,000J

  10. Examples • An apple weighing 1N falls a distance of 1m. How much work is done on the apple by the force of gravity? Given: Fg = 1N d = 1m W = ? W = Fg•d W = 1N x 1m W = 1J

  11. Examples • The brakes of a bicycle apply 125N of frictional force to the wheels as the bicycle travels 14m. How much work have the brakes done on the bike? Given: F = 125N d = 14m W = F•d W = 125N x 14m W = 1750J

  12. Examples • A mechanic uses a hydraulic lift to raise a 1200kg car 0.5m off the ground. How much work does the lift do on the car? Given: m = 1200kg d = 0.5m W = ? W = F•d but… we have a mass, not a force. So, we go back to Fg = mg (g = 9.8m/s/s) F = 1200kg x 9.8m/s/s = 11,760N Now, W = 11,760N x 0.5m = 5880J

  13. Warm up • A father is playing with his baby by lifting her into the air repeatedly. How much work does he do with each lift, assuming he lifts her 2m and exerts an average force of 190N? How much work does he do when he holds her and walks her to her crib?

  14. PSc.3.1.4 • Infer the Work-Power relationship: P = W = FΔd = F x vave

  15. Power • You have to mow 30 lawns in a month. You can mow 6 lawns per day and be done in 5 days or you can mow one lawn per day and take all 30 days to finish. What is analogous to the total number of lawns mowed? Work What is analogous to the rate of mowing? Power

  16. Power • Power is the rate at which work is done. QuickLab: What is your power output when climbing the stairs? Question: Running up a flight of stairs takes the same amount of work as slowly walking up a flight of stairs. Why is it more tiring?

  17. Power • Power = Work = W time t Units: Watts (W) or commonly kW (1kW = 1000W) 1W is the amount of power required to do 1J of work in 1 second.

  18. Example • It takes 100kJ of work to lift an elevator 18m. If this is done in 20s, what is the average power of the elevator during this process? Given: W = 100kJ d = 18 m t = 20s 100kJ x 1000J = 100,000J 1kJ P = Work = 100,000J = 5000W time 20s 5000W x 1kW = 5kW 1000W

  19. Power • You may be familiar with horsepower as a measure of power. This originally referred to the average output of a draft horse. 1hp = 746W

  20. Power • While rowing across the lake in a race, John does 3960J of work on the oars in 60s. What is the power output in watts? Given: W = 3960J t = 60s P = W = 3960 = 66W t 60s

  21. Problems • Suppose you are moving a 300N box of books. Calculate your power output if… • You exert a force of 60N to push the box across the floor 12m in 20s. • You lift the box 1m onto a truck in 3s. For a. W = Fxd = 60N x 12m = 720J P = W/t = 720J/20s = 36W For b. W = Fxd = 300N x 1m = 300J P = W/t = 300J x 3s = 900W

  22. Problems • A student lifts a 12N textbook 1.5m on 1.5s and carries the book 5m across the room in 7s. • How much work does the student do on the book? W = Fxd = 12N x 1.5m = 18J to lift the book Note: No work is done to carry the book across the room because the force on the book (vertical) is in a different direction than the applied force (horizontal). b. What is the power output of the student? P = W/t = 18J/1.5s = 12W

  23. Problems • Compare the work and power used in the following situations: • A 43N force is exerted through a distance of 2.0m over a time of 3s. • A 43N force is exerted through a distance of 3m over a time of 2s.

  24. Problems (cont.) • Given: F = 43N d = 2m t = 3s W = F x d = 43N x 2m = 86J P = W/t = 86J/3s = 29W b. Given: F = 43N d = 3m t = 2s W = F x d = 43N x 3m = 129J P = W/t = 129J/2s = 65W

  25. Energy transformation • http://youtu.be/i6e-KrNCe_E

  26. C. Conservation of Energy • Law of Conservation of Energy • Energy may change forms, but it cannot be created or destroyed under ordinary conditions. • EX: • PE  KE • mechanical  thermal • chemical  thermal

  27. C. Conservation of Energy PE  KE

  28. Conservation of Energy

  29. Check Your Understanding Energy a. Is the ability to do Includes includes can be which measures c. transformed Kinetic energy b. Which is energy which is energy but never acting over a Associated with of distance e. d. position or destroyed

  30. Warm Up • A force of 15N is used to push a box along the floor a distance of 3m. How much work was done? • What is the power of a kitchen blender if it can perform 3,750J of work in 15s?

  31. Warm Up Answers • Given: F = 15N d = 3m W = F x d W = 15N x 3m = 45J 2. Given: W = 3,750J t = 15s P = W/t P = 3,750J/15s = 250W

  32. PSc. 3.1.4 • Determine the component simple machines present in complex machines • Define and determine Ideal Mechanical Advantage • Define and determine the efficiency of machines • Explain why no machine can be 100% efficient

  33. Machines and Mechanical Advantage • Changing a car tire – the jack lets you lift a car you otherwise couldn’t lift • Machines redistribute work – they can change the direction of the input force or they can increase or decrease a force by changing the distance. (W = F x d)

  34. Machines and Mechanical Advantage • Work with and without a machine: You lift a 225N box 1m onto the back of a truck. W = F x d = 225N x 1m = 225J You push the box up a 3m ramp with 75N of force. W = F x d = 75N x 3m = 225J Same amount of work, but the machine (inclined plane) lets you exert much less force.

  35. Machines and Mechanical Advantage • Mechanical advantage tells how much the machine multiplies the force or increases the distance traveled. • Mechanical advantage is the ratio between the output force and the input force or the input distance and the output distance. • AMA = output force IMA = input distance input force output distance

  36. Machines & Mechanical Advantage In the example of lifting a 225N box 1m on to the back of a truck by itself and with a 3m ramp using 75N of force, determine the input and output distances and forces. With ramp: Without ramp: Input F = 75N Input F = 225N Input d = 3m Input d = 1m Output F = 225N Output F = 225N Output d = 1m Output d = 1m

  37. Machines and Mechanical Advantage • No machine can increase force and distance at the same time: W=fxd if one increases the other must decrease. (See example above) • Said another way, you can’t get more work out of a machine than you put into it. • Machines don’t increase the amount of work being done; they make the work easier to do.

  38. Problems • A mover uses a pulley system with a mechanical advantage of 10.0 to lift a piano 3.5m. Disregarding friction, how far must the mover pull the rope? MA = input dist output dist 10.0 = input dist/3.5m Input dist = 10.0 x 3.5m = 35m

  39. Problems • A person pushes a 950N box up an incline. If the person exerts a force of 350N along the incline, what is the mechanical advantage of the incline? • MA = output force = 950N = 2.7 input force 350N What are the units of mechanical advantage? None, it is a ratio. The units cancel out.

  40. Problems • Calculate the MA of a ramp that is 6m long and 1.5m high. • A sailor uses a rope and pulley to lift a 140N sail. The sailor pulls down with a force of 140N on the rope. What is the MA of the pulley? • Alex pulls on the handle of a claw hammer with a force of 15N. If the hammer has a MA of 5.2, how much force is exerted on the nail in the claw? • A rower pulls an oar back a distance of 0.8m on each stroke. If the oar has a MA of 1.5, how far does the blade of the oar move through the water on each stroke?

  41. MA = input dist/output dist = 6.0m/1.5m = 4.0 • MA = output force/input force = 140N/140N = 1 • Output force = MA x input force = 5.2 x 15N = 78N • Output dist = input dist/MA = 0.8m/1.5 = 0.53m

  42. Warm Up • Put your phones in your backpacks and get out your notes and a pencil. 2. Write the term with its correct definition. Then write its equation and its units. • Work • Power • Mechanical advantage • The amount that a machine multiplies a force or a distance • The rate at which work is done • What is done when a force makes an object move

  43. Simple Machines 6 types of simple machines classified in 2 groups: The Lever FamilyThe Inclined Plane Family Simple lever Simple inclined plane Wheel & Axle Wedge Pulley Screw

  44. The Lever Family 1st Class Lever Fulcrum is between points of application of input and output forces. Fulcrum – point on which the lever balances Input Force – the force you apply to the lever (effort) Output Force – the force the lever exerts on the object (load) Ex: claw hammer, teeter totter

  45. The Lever Family 2nd Class Lever Fulcrum is at one end of the arm and input force is applied to the other end. Ex: wheelbarrow – wheel is fulcrum nutcrackers hinged doors

  46. The Lever Family 3rd Class Levers Multiply distance rather than force. Ex: many in human body – bicep contracts a small distance and the hand moves a large distance Which is the bicep? Which is the hand? Which is the fulcrum?

  47. The Lever Family • 1st Class Levers – can either multiply force OR increase distance (not both) claw hammer • 2nd Class Lever – always multiplies force hinged door • 3rd Class Lever – always increases distance broom

  48. The Lever Family Pulleys Ex: flag pole, sail on boat Can have different set ups of fixed and free pulleys that change the input force required.

  49. Pulleys Lifting a 150N weight with a single fixed pulley, the weight must be fully supported by the rope on each side of the pulley. (MA=1) Mechanical Advantage in pulley systems is found by counting the number of weight supporting ropes. Input Force = 150N Output Force = 150N

  50. Pulleys Output force = 150N Input force = 75N The 150N force is shared by two sections of rope both pulling upward. (MA=2)