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Work and machines

Work and machines. Making jobs easier. Work. Work is done on an object when a force is exerted on an object that causes the object to move some distance. No work without motion No work without force in the same direction. Person does not work on weights – weights do not move.

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Work and machines

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  1. Work and machines Making jobs easier

  2. Work • Work is done on an object when a force is exerted on an object that causes the object to move some distance. • No work without motion • No work without force in the same direction

  3. Person does not work on weights – weights do not move Person does work on weights – weights moved Force on weights Force on weights Motion and force Summer Olympics 2000 Sydney Australia By Lou Jones http://www.pricespower.com

  4. Horse does not work on the rider – force not in same direction Horse does work on the cart - force and motion in the same direction Direction and force Force on person Force on cart motion motion

  5. Calculating work • Work = Force X Distance • Force units = Newton, N • Distance units = meter, m • Work units = Joule, J • J = N*m

  6. Making work easier • Machines • Change the amount of force you exert • Change the distance over which you exert force • Change the direction in which you exert force • Do change the amount of work done

  7. Total work does not change • Work = Force X Distance • If a machine allows you to apply a smaller force the distance will increase • If a machine reduces the distance the force will increase • If a machine allows you to apply a force in a different direction it doesn’t change how much force you have to apply or how far you move it

  8. Mechanical advantage • The number of times a force exerted on a machine is multiplied by the machine • Mechanical ad. = Output force Input force • If M.A. > 1 machine increases force needed but distance decreases • If M.A. < 1 machine reduces force needed but distance increases

  9. Mechanical advantage output force (in newtons) Mechanical advantage ------------------------------ input force(in newtons)

  10. Mechanical advantage output force (in newtons) Mechanical advantage ------------------------------ input force(in newtons) • an input force of 202 N is applied to the handles of the wheelbarrow in the sample problem, how large is the output force that just lifts the load? • Suppose you need to remove a nail from a board by using a claw hammer. What is the input distance for a claw hammer if the output distance is 2.0 cm If and the mechanical advantage is 5.5?

  11. Efficiency • Efficiency compares the output work to the input work (%) • Cannot be greater than 100% • Efficiency usually < 100% • Friction

  12. Efficiency output work (in newtons) Efficiency advantage ------------------------------ input work(in newtons) • Wile E. Coyote sets up a pulley to lift a gigantic boulder over a valley highway in an attempt to squish the Roadrunner. Wile E. Coyote puts 3402 J of energy into the pulley while the pulley does 2938 J of work. Calculate the efficiency of the pulley. • 2. Wile E. Coyote’s previous attempt of killing the Roadrunner failed so he decided to use an inclined plane to push a boulder down onto the Roadrunner. The output work done by the inclined plane is 875 J and the input work is 1285 J. Calculate the efficiency of the inclined plane.

  13. Types of simple machines • Inclined plane • Wedge • Screw • Lever • Wheel and axle • pulley Image taken from: http://www.daniel-wright.district103.k12.il.us/ccheifetz/physics.html

  14. Inclined plane teacher.scholastic.com/dirtrep/ simple/img/plane.gif

  15. Wedge teacher.scholastic.com/dirtrep/ simple/img/wedge.gif

  16. Screw teacher.scholastic.com/dirtrep/ simple/img/screw.gif

  17. Lever teacher.scholastic.com/dirtrep/ simple/img/lever.gif

  18. Wheel and axle teacher.scholastic.com/dirtrep/ simple/img/wheel.gif

  19. Pulley teacher.scholastic.com/dirtrep/ simple/img/pulley.gif

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