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  1. Work, Power, and Machines Physical Science – Unit 7 Chapter 9

  2. Work • What is work? • Work is the quantity of energy transferred by a force when it is applied to a body and causes that body to move in the direction of the force. • Examples: • Weightlifter raises a barbell over his/her head • Using a hammer • Running up a ramp

  3. Work Work in simple terms: • Transfer of energy that occurs when a force makes an object move • The object must move for work to be done • The motion of the object must be in the same direction as the applied force

  4. Work • The formula for work: • Work = force x distance • W = F x d • Measured in Joules (J) • Because work is calculated as force times distance, it is measured in units of newtons times meters (N●m) • 1 N●m = 1 J = 1 kg●m2/s2 • They are all equal and interchangeable! James Joule - English scientist and inventor 1818-1889

  5. Work • 1 J of work is done when 1N of force is applied over a distance of 1 m. • kJ = kilojoules = thousands of joules • MJ = Megajoules = millions of joules

  6. Practice problem A father lifts his daughter repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? W = F x d

  7. Practice problem A father lifts his daughter repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? W = F x d W = 190 N x 2.0 m = 380 N●m = 380 J

  8. Practice problems A mover is moving about 200 boxes a day. How much work is he doing with each box, assuming he lifts each 10 m with a force of 250 N.

  9. Practice problems A mover is moves about 200 boxes a day. How much work is he doing with each box, assuming he lifts each 10 m with a force of 250 N. W = F x d = 250 N x 10 m = 2,500 N●m = 2,500 J

  10. Practice problems A box with a mass of 3.2 kg is pushed 0.667 m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box? What do you need to calculate first?????

  11. Practice problems A box with a mass of 3.2 kg is pushed 0.667 m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box? F = ma = 3.2 kg x 3.2 m/s2 =10.2 kg● m/s2 = 10.2 N

  12. Practice problems A box with a mass of 3.2 kg is pushed 0.667 m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box? F = ma = 3.2 kg x 3.2 m/s2 =10.2 kg● m/s2 = 10.2 N W = F x d = 10.2 N x 0.667 m = 6.80 N●m = 6.80 J

  13. Practice problems

  14. Power • Power is a quantity that measures the rate at which work is done • It is the relationship between work and time • If two objects do the same amount of work, but one does it in less time. The faster one has more power. • Rate at which work is done or how much work is done in a certain amount of time

  15. Power • Formula for power: Power = work time P = W/t • SI units for power – watts (W) • 1 kW – Kilowatt = 1000 watts • 1 MW – Megawatt= 1 million watts

  16. Power • A watt is the amount of power required to do 1 J of work in 1 s. (Reference – the power you need to lift an apple over your head in 1 s) • Named for James Watt who developed the steam engine in the 18th century.

  17. Practice problems A weight lifter does 686 J of work on a weight that he lifts in 3.1 seconds. What is the power with which he lifts the weight? P = W/t

  18. Practice problems A weight lifter does 686 J of work on a weight that he lifts in 3.1 seconds. What is the power with which he lifts the weight? P = W = 686 J t 3.1 s 221 J/s = 221 W

  19. Practice problems • How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? P = W t

  20. Practice problems • How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? P = W t 1st convert 8 hr to seconds 8 hr (60 min/1hr)(60 sec/1min) = 28800 sec

  21. Practice problems • How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? P = W t 1st convert 8 hr to seconds 8 hr (60 min/1hr)(60 sec/1min) = 28800 sec 2nd calculate for energy W = P x t = 60 W x 28800 sec = 1.7 x 107 J

  22. Practice problems

  23. Machines and Mechanical Advantage • Which is easier… lifting a car yourself or using a jack? • Which requires more work? • Using a jack may be easier but does not require less work. • It does allow you to apply less force at any given moment.

  24. What is a machine? • A device that makes doing work easier… is a machine • Machines increase the applied force and/or change the direction of the applied force to make the work easier • They can only use what you provide!

  25. Why use machines? • If machines cannot make work, why use them? • Same amount of work can be done by applying a small force over a long distance as opposed to a large force over a small distance.

  26. Effort and Resistance • Machines help move things that resist being moved • Force applied to the machine is effort force (aka: Input force) • Force applied by the machine is resistance force (aka: Load)

  27. Mechanical Advantage • Mechanical advantage is a quantity that measures how much a machine multiplies force or distance • Defined as the ratio between output force and input force

  28. Mechanical Advantage • Formula: • Mechanical advantage = output force input force Or mechanical advantage= input distance output distance

  29. Practice problems • A roofer needs to get a stack of shingles onto a roof. Pulling the shingles up manually used 1549 N of force. Using a system of pulleys requires 446 N. What is the mechanical advantage? Mechanical advantage = output force input force

  30. Practice problems • A roofer needs to get a stack of shingles onto a roof. Pulling the shingles up manually used 1549 N of force. Using a system of pulleys requires 446 N. What is the mechanical advantage? Mechanical advantage = output force input force = 1549 N = 3.47 446 N

  31. Practice problems

  32. Machines Simple Machines • Lever • Pulley • Wheel & Axle • Inclined Plane • Screw • Wedge

  33. The Lever family Lever a rigid bar that is free to pivot about a fixed point, or fulcrum Force is transferred from one part of the arm to another. Resistance arm Effort arm Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes

  34. Lever First Class Lever Most common type can increase force, distance, or neither changes direction of force

  35. Lever Second Class Lever always increases force

  36. Lever Third Class Levers always increases distance

  37. Pulley Pulley grooved wheel with a rope or chain running along the groove a “flexible first-class lever” or modified lever F Le Lr

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

  39. Pulley Fixed Pulley • IMA = 1 • does not increase force • changes direction of force

  40. Pulley Movable Pulley • IMA = 2 • increases force • doesn’t change direction

  41. Pulley Block & Tackle • combination of fixed & movable pulleys • increases force (IMA = 4) • may or may not change direction

  42. Wheel and Axle Wheel and Axle two wheels of different sizes that rotate together a pair of “rotating levers” When the wheel is turned so so is the axle Wheel Axle

  43. Wheel and Axle Wheel and Axle Bigger the difference in size between the two wheels= greater MA Wheel Axle

  44. What is an inclined plane? • A sloping surface, such as a ramp. • An inclined plane can be used to alter the effort and distance involved in doing work, such as lifting loads. • The trade-off is that an object must be moved a longer distance than if it was lifted straight up, but less force is needed.

  45. What is an inclined plane? • MA=Length/Height

  46. Incline Plane Family • A wedge is a modified incline plane • Example ax blade for splitting wood • It turns a downward force into two forces directed out to the sides

  47. Incline Plane Family • A screw looks like a spiral incline plane. • It is actually an incline plane wrapped around a cylinder • Examples include a spiral staircase and jar lids

  48. Compound Machines • Compound machines are machines made of more than one simple machine • Example include a pair of scissors has 2 first class levers joined with a common fulcrum; each lever arm has a wedge that cuts into the paper