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Chapter 5

Chapter 5. Work and Machines. Work . Transfer of energy that occurs when a force makes an object move. Work – Two Conditions. 1. The object must move. 2. The motion must be in the same direction as the applied force. Is Work Done?. Picking up a stack of books. Holding a stack of books.

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Chapter 5

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  1. Chapter 5 Work and Machines

  2. Work • Transfer of energy that occurs when a force makes an object move.

  3. Work – Two Conditions • 1. The object must move. • 2. The motion must be in the same direction as the applied force.

  4. Is Work Done? • Picking up a stack of books. • Holding a stack of books. • Walking while holding a stack of books.

  5. Work and Energy • When work is done, a transfer of energy occurs (energy = ability to do work) • Ex. When you pick up a box, you transfer energy from your muscles to the box, increasing its PE

  6. Calculating Work • Work = Force X distance • W=Fd • F = N • d = m • W = N-m = J

  7. Power • Is the rate at which work is done. • Amount of work done in a certain amount of time.

  8. Calculating Power • Power = Work/time • P = W/t • W = J • t = sec • P = J/sec = Watts (W)

  9. Power and Energy • Energy can be transferred without involving work. • Ex. Light bulb transfers energy into heat and light. • P = E/t

  10. Machines • A device that makes doing work easier. • Work by: • Increasing F (car jack) • Increasing d (ramp) • Changing direction (ax)

  11. Work done by Machines • Two forces involved • Effort Force (Fe) – the force applied to a machine. • Resistance Force (Fr) – the force applied by the machine to overcome resistance.

  12. Work done by Machines • Two types of work involved: • Input work (Win) – work done by you on a machine. • Output work (Wout) – work done by the machine.

  13. Conserving Energy • What does the Law of Conservation of Energy state? • You transfer energy to the machine, and the machine transfers energy to an object.

  14. Conserving Energy • A machine cannot create energy, so Wout is never greater that Win. • Can Wout = Win ? Why?

  15. Ideal Machines • What is an ideal Machine? • If a machine allows you to apply less force, then how do you get the same amount of work out of the machine?

  16. Mechanical Advantage (MA) • It is the number of times a machine multiplies the effort force. • IMA vs AMA

  17. Calculating MA • MA = resistance force/effort force • MA = Fr/Fe • MA has no units. Why?

  18. Efficiency • A measure of how much of the work put into a machine is converted into useful output work. • Why is work output always less than work input? • How can you  efficiency?

  19. Calculating Efficiency • Efficiency = (Wout/Win) X 100%

  20. Simple Machines • A machine that does work with only one movement. • Six Types: 1)lever, 2)pulley, 3)wheel and axle, 4) inclined plane, 5) screw, and 6) wedge.

  21. Lever • A bar that is free to pivot about a fixed point or fulcrum. • Effort arm = distance from fulcrum where effort force is applied. • Resistance arm = distance from fulcrum where resistance force is applied

  22. Types of Levers • Based on positions of effort force, resistance force, and fulcrum.

  23. Types of Levers • 1st class – fulcrum is in the middle (crowbar) • 2nd class – resistance is in the middle (wheelbarrow) • 3rd class – effort is in the middle (baseball bat)

  24. IMA of Levers • IMA = length of effort arm/length of resistance arm • IMA = Le/Lr

  25. Pulleys • A grooved wheel with a rope, chain or cable running along the groove. • How is a pulley a modified 1st class lever?

  26. Fixed Pulleys • Attached to something that does not move • Only change the direction of the force • IMA = 1

  27. Moveable Pulleys • One end of the rope is fixed and the wheel is free to move. • Multiplies effort force • IMA = 2

  28. Block and Tackle • A system of pulleys made of fixed and moveable pulleys • IMA = # of supporting rope segments. Only count effort segment if effort force is in the direction of the object’s movement.

  29. Wheel and Axle • Machine consisting of two wheels of different sizes that rotate together. • Modified form of a lever. • IMA = rw/ra

  30. Gears • Modified Wheel and Axle with two wheels of different sizes with interlocking teeth. • Large wheel = effort gear, small wheel = resistance gear. • Larger effort gear = more turns of resistance gear =  effort force

  31. Inclined plane • Sloping surface that reduces the amount of force required to do work. • Increases distance • IMA = length/height

  32. Screw • Inclined plane wrapped, in a spiral, around a cylinder.

  33. Wedge • Moving inclined plane with one or two sloping sides. • Changes the direction of effort force

  34. Compound Machines • When two or more simple machines are used together. • Examples?

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