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Bellringer

Bellringer. Compare and explain in complete sentences what is work. Homework. CALCULATE KINETIC ENERGY OF AN OBJECT DROPPED FROM A BUILDING 10 M HIGH, MASS 50 kg JUST BEFORE IT HITS THE GROUND. Work, Power & Machines. Objectives. Define work and identify the units

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Bellringer

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  1. Bellringer Compare and explain in complete sentences what is work

  2. Homework CALCULATE KINETIC ENERGY OF AN OBJECT DROPPED FROM A BUILDING 10 M HIGH, MASS 50 kg JUST BEFORE IT HITS THE GROUND

  3. Work, Power & Machines

  4. Objectives • Define work and identify the units • Describe the conditions that must exist for a force to do work on an object • Calculate the work done on an object • Describe and calculate power • Compare the units of watts and horsepower as they relate to power

  5. Work • Definition - quantity of energy transferred by a force when it is applied to a body and causes that body to move in the direction of force • Formula - W = F x D • Units - Newton meter (N/m) or a joule (J)

  6. Work Continued • Two factors 1. size of force, and direction it is applied ex: pulling a suitcase * any part of a force that does not act in the direction of motion does no work on an object 2. movement of something by that force

  7. Work Cont. • A weight lifter who holds a barbell weighing 1000 N does NO work • Why? - he may get very tired, but if the barbell is not moved by the force he exerts, he does no work on the barbell - work is done on his muscles when he raises the barbell

  8. Work Problems Q: A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder? A: W = (5200N)(25m) = 1.3 x105 J Q: While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes? A: W = (30)(165N)(0.800m) = 3960 J 4.0 x 103

  9. Work Problems Cont. Q: Jake, a 235 N track athlete completes his race, which totals 1575 J, what is the total distance Jake ran? A: 1575 J = 6.70 m 235 N Q: Joey, performed 900 J of work, while lifting a box 12 meters. What force did Joey exert on the box? A:75 N

  10. Power • Definition - a quantity that measures the rate at which work is done • Formula - P = W/t • Unit - Joule per second (J/s) or Watts (W) - Horsepower (Hp) = 746 W

  11. Power Problems Q: Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output? A: P = 5350J/ 50.0s = 107 W Q: Anna walks up the stairs on her way to class, She weighs 565 N and the stairs go up 3.25 m vertically. Calculate the power output if she climbs the stairs in 12.6 s. What is her power output if she climbs the stairs in 10.5 s? A: P = (565N)(3.25m)/12.6s =146 W P = (565N)(3.25m)/10.5s = 175 W

  12. Objectives • Describe what a machine is and how it makes work easier • Relate the work input to a machine to the work output of the machine • Compare a machines actual mechanical advantage to its ideal mechanical advantage • Calculate the ideal and actual mechanical and actual mechanical advantages of various machines • Explain why efficiency of a machine is always less than 100% • Calculate the machines efficiency

  13. Machines • Definition - a device that changes force ex. jack, nutcracker, an oar • How does a machine change force? - 3 ways a machine makes work easier to perform - change the size of force needed - the direction of a force - the distance over which a force acts

  14. Changing the Force • Increasing the force you applied ex. jack applied to a car - small force exerted over a large distance becomes a largeforce exerted over a shortdistance

  15. Changing the Distance • Increasing distance ex. rowing a boat with oars • small movement of oar at the hands makes a large distance the oar in the water will move. *remember the trade off: small distance large force*

  16. Changing the Direction • Changing the direction of the applied force ex. rowing a boat - pulling back on the handle of the oars causes its other end to move in the opposite direction.

  17. Work Input • Work Input - the workdone on a machine as the inputforce acts through the inputdistance - input force: force exerted on a machine - input distance: distance the input force acts through - equals the input force multiplied by the input distance ex. rowing a boat - input distance < output distance - input force > output force

  18. Work Output • Work Output - the work done by a machine as the output force acts through the output distance - output force: force exerted by a machine - output distance: distance the output force is exerted through *due to friction the work done by a machine is always less than the work done on the machine

  19. Mechanical Advantage • Definition - a quantity that measures how much a machine multiplies force or distance • Two types • Actual: measures the actual forces action on a machine - AMA = output force input force • Ideal: measures the mechanical advantage in the absence of friction - IMA = input distance output distance

  20. Mechanical Advantage Problems Q: Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a actual mechanical advantage of 5.2, how much force is exerted on a nail in the claw? A: output force = (5.2)(15N) = 78 N Q: If you exert 100 N on a jack to lift a 10,000 N car, what would be the jack’s actual mechanical advantage (AMA) A: AMA= 10,000 N / 100 N = 100

  21. Mechanical Advantage Problems Q: Calculate the ideal mechanical advantage (IMA) of a ramp that is 6.0 m long and 1.5 m high? A: IMA = 6.0m / 1.5m = 4.0 Q: The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance? A: Input distance = (2.5)(1.0m) = 2.5 m

  22. Efficiency of Machines • Definition - a quantity, usually expressed as a percentage, that measures the ratio of useful work input • Formula - Efficiency = useful work output work input - % of work input that becomes work output - due to friction, efficiency of any machine is always less than 100%

  23. Efficiency Problems Q: Alice and Jim calculate that they must do 1800 J of work to push a piano up a ramp. However, because they must also overcome friction, they must actually do 2400 J of work. What is the efficiency of the ramp? A: 1800 J/ 2400 J x 100 = 75% Q: If the machine has an efficiency of 40%, and you do 1000 J of work on the machine, what will be the work output of the machine? A: Work Output = (Efficiency x work input) / 100% Work Output = (40% x 1000 J) / 100% = 4.0 x 102 J

  24. Objectives • Name, describe and give an example of each of the six types of simple machines • Describe how to determine the ideal mechanical advantage of different types of simple machines • Define and identify compound machines • Recognize simple machines within compound machines

  25. Simple Machines • Definition - one of the six basic types of machines • 2 types or families 1. lever 2. inclined planes

  26. Levers • Definition - a rigid bar that is free to move around a fixed point ex. screwdriver • all levers have a rigid arm that turns around a point called the fulcrum • force is transferred from one part of the arm to another • original input force can be multiplied or redirected into output force • levers are divided into 3 classes, based on the locations of the input force, output force, and the fulcrum

  27. Lever Family Cont. • 3 classes 1. First class - fulcrum is in the middle of an arm - the input force acts on one end - the other end applies an output force - MA can be: <1, >1, =1 ex. teeter totter, scissors, tongs 2. Second class - fulcrum is at one end of the arm - input force is applied to the other end - output force is in the middle - MA will always > 1 ex. wheel barrow

  28. Lever Family Cont. 3. Third class - input force is in the middle - output force is one one end - fulcrum is on the other end - multiplies distance rather than force MA always < 1 ex. baseball bats, hockey sticks, golf clubs, human body

  29. Wheel and Axis • Definition - simple machine that consists of two disks or cylinders, each one with a different radius ex. steering wheel, screwdriver • made of a level or a pulley (wheel) connected to a shaft (axle) • small input force, multiplied to become a large output force

  30. Inclined Planes • Definition - slanted surface along which a force moves an object to a different elevation ex. knife, ax, zipper, wedge, screw • ramp redirects the force applied to lift object upward • turns a small input force into a large output force by spreading the work out over a large distance - wedge: functions as two inclinedplanes back to back, turning a downward force into two forces directed out to the sides -screw: an inclined plane wrapped around a cylinder

  31. Pulleys • Definition - a simple machine that consists of a rope that fits into a groove in a wheel. - very similar to a lever • point in the middle of the pulley is like a fulcrum • rest of the pulley acts like the rigid arm • 3 Types of Pulleys - fixed pulleys - moveable pulleys - pulley system

  32. Types of Pulleys • fixed pulleys: changes the direction of the input force • moveable pulleys: changes both the direction and the size of the input force • pulley system: made of both fixed and moveable pulleys

  33. Compound Machines • Definition - a machine that is made of more than one simplemachine ex. scissors, jacks, bicycle, washing machine,car, clock

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