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Chapter 5

Chapter 5. Work and Machines. What is work?. Work is done when an applied force causes an object to move 2 things must be true for work to be done: An applied force makes an object move The direction of the force is the same as the direction that the object moves. Who is doing more work?.

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Chapter 5

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  1. Chapter 5 Work and Machines

  2. What is work? Work is done when an applied force causes an object to move 2 things must be true for work to be done: • An applied force makes an object move • The direction of the force is the same as the direction that the object moves

  3. Who is doing more work?

  4. Is work being done on the books? • When you carry books, the force on the books is upward and your motion is horizontal so work is NOT being done on the books.

  5. How are work and energy related? • When work is done, energy is transferred from one form to another. • Energy is the ability to do work. • (more on energy in chapter 4…)

  6. Calculating Work Work is measured with the unit, joules Work (joules) = applied force (newtons) x distance (meters) W = Fd

  7. Practice 1. A couch is pushed with a force of 75 N and moves a distance of 5 m across the floor. How much work is done in moving the couch?

  8. POWER

  9. Power • Power is a measure of the amount of work done in 1 second - The rate at which work is done - The rate at which energy is transferred

  10. Practice 2. If a runner’s power is 130 W as she runs, how much work is done by the runner in 10 minutes?

  11. Power and Energy • Doing work is a way of transferring energy from one object to another • Power is the rate at which work is done & the rate at which energy is transferred

  12. Calculating Power

  13. Machines Page 133: figure 6, figure 7, figure 8 (check these out during your “15 minutes”) A machine is a device that makes work easier to do by: • Increasing force • Increasing distance • Changing direction

  14. The work done by machines • Involves 2 forces: • input force (Fin): you exert • output force(Fout): machine exerts • Kinds of work • work done on the machine (Win) • work done by the machine (Win )

  15. The work done by machines • Conserving energy • Win is never > Wout • Energy can not be created… • Ideal machines • Win = Wout • …or destroyed… • Real machines • Some energy is lost as heat due to friction when the machine performs work • Woutis always smaller than Win • …only changed in form

  16. Mechanical Advantage • Machines make the output force from the machine greater than the input force from the person using the machine. • Mechanical Advantage (MA) is the ratio of output force (Fout) to input force (Fin)

  17. Efficiency • The efficiency of a machine is the ratio of force you get out of the machine vs. the amount of force input by the machine. x 100 • The more efficient a machine is, the less heat it will generate. • Lubricating a machine can make it more efficient.

  18. Simple Machines • Simple machines do work with only one movement.

  19. Levers Fout in the opposite direction of Fin Fout is greater than Fin Fout is less than Fin, dout is greater than din

  20. Ideal mechanical advantage of a lever Ideal mechanical advantage =

  21. Pulley • Fixed Pulley – changes the direction of the force • IMA = 1 • Moveable Pulley – multiplies force • IMA=2 • Block and tackle – a system of pulleys • IMA = # rope segments

  22. Wheel and Axle

  23. Gears • Gears are wheels with interlocking teeth • Gears of different sizes rotate at different rates • Gears can also change direction of force • IMA is same as wheel & axle

  24. Inclined Plane An inclined plane works by increasing the distance an object moves so that less force will be needed to complete the work

  25. Screw • A screw is a spiral shaped inclined plane • IMA is related to spacing of laces; IMA is larger if spacing is smaller

  26. Wedge A wedge is also a type of inclined plane. Wedges are used to cut through objects. Changes the direction of the force

  27. Compound Machine How many simple machines are found in a fishing pole?

  28. Compound Machines • Page 146 figure 23 Rube-Goldberg Device – http://www.youtube.com/watch?v=SwaTKYJSDaw http://www.youtube.com/watch?v=YWk9N92-wvg&list=FLUY9NuaaCOjd26ObQt0O53A&index=1

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