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Physical Science Unit

Understand the concept of work in physical science and learn about machines and their efficiency. Solve word problems and calculate work, force, power, and mechanical advantage.

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Physical Science Unit

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  1. Physical ScienceUnit Work & Machines

  2. What is Work? Work = Force x Distance SI Unit for work is the Joule 1 Joule = 1Newton x 1 Meter Workis force exerted on an object that causes the object to move some distance Force without moving a distance yields NO WORK!!

  3. 2 Conditions must be met in order for work to be done: 1. Force must be applied 2. Force must make an object move in the same direction as the force Work does not involve time Work only involves an applied force and movement of an object in the direction of the force

  4. Word Problems • Word problems can be confusing; but w/ some practice they’re not that bad. Here are a few hints to make them easier • 1. Be sure you remember the “Need-to-Know” formulas • S =d/t ; A = Vf – Vi ; F = MA ; W=FxD; Power = Work/Time Time • In the word problem be sure you know the units for each of the variables in the particular formula being discussed. • Distance – Meter; Force – Newton; Volume - cm3 or Liter • 2. In the word problem, all but one of the variables is told to you in one way or another. Identify what variable is being asked to solve, then plug in the remaining variables to the formula • Solve it!! Make sure you also keep track of the units

  5. Measuring work: Work is measured in Newton-meters (N-m) 1N-m = 1 joule = 1 unit of work The force that is applied to do work is called effort force The force that resists motion is called resistance force

  6. How much work performed: • How much work is performed if you apply 85 newtons of force on a box causing it to move 3 meters: W = F x D W = 85N x 3m = 255 Nm 255 J = 255 Nm • How much work is performed if you apply 37 newtons of force and move a wagon 4.3 meters? W = F x D W = 37N x 4.3m = 159.1 Nm 159.1 J = 159.1 Nm • How much work is performed if you apply 118 newtons of force on a car that is stuck in the mud and doesn’t move?: W = F x D W = 118N x 0m = 0 Nm 0J =0Nm You might be tired from pushing but no work was done!!

  7. How much force required: • How much force was required to move an object 3 meters if 75 Joules of work were expended? • Formula: Work = Force x Distance • Need to solve for Force, w= 75 J & D=3M 75 J = F x 3M 75 NM / 3M = F 75 NM / 3M = F 25N = F

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

  9. Power • Time is involved • Power is measured in watts (w) • Unit of power is N-m per second or joule per second (watt) • 1000watts =I kilowatt • 750 watts = I horsepower

  10. Horse Power • Since the horse was the most common source of power in the 18th century, James Watt decided to express the steam engine power in terms of comparison to the power of a horse

  11. Horse Power • 1 horsepower = 746 watts

  12. Power

  13. W P t Power • A figure skater lifts his partner, who weighs 450 N, 1.0 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

  14. Machines • Machine • device that makes work easier • changes the size and/or direction of the exerted force

  15. When a machine is used, 2 forces are always involved: • Effort Force (Fe) • force applied to the machine • “what you do” • Resistance Force (Fr) • force applied by the machine • “what the machine does”

  16. Force Effort Force FE Resistance Force FR

  17. Work • Work Input (Win) • work done on a machine Win = Fe × de • Work Output (Wout) • work done by a machine Wout = Fr × dr

  18. Work • While machines make work easier, they do not multiply work, they only multiply force. • So work output can never be greater than work input

  19. Work • Conservation of Energy • can never get more work out than you put in • trade-off between force and distance Win = Wout Fe × de = Fr × dr

  20. Efficiency • Efficiency • measure of how completely work input is converted to work output • always less than 100% due to friction

  21. Efficiency • The efficiency of a machine can never be greater than 100% because work output can never be greater than work input

  22. Efficiency • If a machine has a high efficiency, it means that much Work input has changed to work output • If there is a low efficiency, It means that much work input is lost

  23. 4.0m 500N 1.0m 1500N Efficiency • 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: Fe = 500 N de = 4.0 m Fr = 1500 N dr = 1.0 m WORK: Win = (500N)(4.0m) = 2000 J Wout = (1500N)(1.0m) = 1500 J E = 1500 J × 100% 2000 J E= 75%

  24. Work • In an ideal machine... Win = Wout • But in the real world… • some energy is lost as friction Win > Wout

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

  26. Fr Fe MA Mechanical Advantage • A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA? GIVEN: Fe = 20 N Fr = 500 N MA = ? WORK: MA = Fr ÷ Fe MA = (500 N) ÷ (20 N) MA = 25

  27. Fr Fe MA Mechanical Advantage • Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: Fe = ? Fr = 2000 N MA = 10 WORK: Fe = Fr ÷ MA Fe = (2000 N) ÷ (10) Fe = 200 N

  28. M.A. • A machine’s mechanical advantage is the number of times a force exerted on a machine is multiplied. • Ideal Mechanical Advantage has no units ( they cancel each other out when doing the math problem • IMA = output force / input force

  29. Simple Machines: 6 types

  30. Inclined Plane A slanted surface used to raise an object The M.A. of an inclined plane is the length divided by the height Because the length of the plane can never be shorter than its height, the M.A. of an inclined plane can never be less than one

  31. Inclined Plane A common inclined plane is a ramp. Lifting a heavy box onto a loading dock is much easier if you slide the box up a ramp--a simple machine.

  32. h l D. Inclined Plane • Inclined Plane • sloping surface used to raise objects

  33. Fr l Fe h MA IMA 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: Fe = ? Fr = 450 N l = 3 m h = 1.2 m WORK: IMA = l ÷ h IMA = (3 m)÷(1.2 m) IMA = 2.5 Fe = Fr ÷ MA Fe = (450 N)÷(2.5) Fe = 180 N

  34. Wedge you can use the edge of an inclined plane to push things apart. Then, the inclined plane is a wedge. So, a wedge is actually a kind of inclined plane. An axe blade is a wedge. Think of the edge of the blade. It's the edge of a smooth slanted surface.

  35. Wedge • Wedge • a moving inclined plane with 1 or 2 sloping sides The longer and thinner the wedge, the less FE Required to overcome the FR

  36. Wedge • Zipper • 2 lower wedges push teeth together • 1 upper wedge pushes teeth apart

  37. Screw • an inclined plane wrapped around a cylinder • A screw can convert a rotational force (torque) to a linear force and vice versa.

  38. Screw • Screw • inclined plane wrapped in a spiral around a cylinder • It multiplies an FE by acting thru a long effort distance The closer the threads, the greater the M.A.

  39. Lever Any tool that pries something loose is a lever.

  40. Resistance arm Effort arm Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes A. Lever • Lever • a bar that is free to pivot about a fixed point, or fulcrum

  41. Effort arm length Resistance arm length A. Lever • Ideal Mechanical Advantage (IMA) • frictionless machine • Le must be greater than Lr in order to multiply the force.

  42. 1st Class Levers • Notice how • The input & output forces are in opposite directions • The fulcrum is between the input & output forces • Examples include nail remover, paint can opener scissors, seesaw

  43. A. Lever • First Class Lever • can increase force, distance, or neither • changes direction of force

  44. 2nd Class Levers • Notice how: • The input & output forces are in the same direction • Input force is farther away from the fulcrum than the output force • Examples include: wheel barrow, door, nutcracker

  45. A. Lever • Second Class Lever • always increases force

  46. 3rd Class Lever • Notice how: • The input & output forces are in the same direction • The input force is closer to the fulcrum than the output force • Examples include rake, shovel, baseball bat and fishing pole

  47. A. Lever • Third Class Levers • always increases distance

  48. What Class of Lever? 3 2 1 4 5 6 7 • _______ 2. _______ 3. _______ 4. _______ • 5. _______ 6. _______ 7. _______ 8. _______ • 3rd Class 2. 1st Class 3. 1st Class 4. 2nd Class • 5. 2nd Class 6. 3rd Class 7. 1st Class 8. 2nd Class 8

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