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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. The unit of work is the Joule • Force x Distance = Work • 1 Newton x 1 meter = 1 Newton meter • 1 Newton meter = 1 Joule James Prescott Joule

  7. Example 1: • A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high? • Answer: W = F x d • W = 700N x 2.0 m • = 1400 nm • = 1400 Joules

  8. No work is done if there is no distance! • The statue of liberty has been holding up her torch for an awfully long time. How much work has she done? • Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done.

  9. 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 NOT

  10. 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

  11. 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

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

  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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