Work and power


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Work and power

  1. 1. Work and Power
  2. 2. Work Work is the transfer of energy that occurs when a force makes an object move. 2 conditions for work to be done  An applied force must make the object move  Movement must be in the same direction as the force. Force Movement
  3. 3. Work How is the scientific definition of work different from the every day meaning? Force Movement
  4. 4. Work When work is done transfer of energy always occurs. When you work on an object, you increase its energy.
  5. 5. Calculating Work Work = applied force * distance Or w=Fd Create a calculation triangle to make it easier to solve for “w”. W F x d
  6. 6. Calculating Work cont. A force of 75N is exerted on a 45 kg couch and the couch is moved 5m. How much work is done in moving the couch? The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50m while the brakes are being applied, what is the total force of the brakes on the car?
  7. 7. Power Power is the amount of work done in one second. Rate at which work is done
  8. 8. Calculating Power Power = work/ time Create a calculation triangle to make it easier to solve for “p”. W P x t
  9. 9. Calculating Power cont. To lift a baby from a crib 50 J or work are done. How much power is needed to lift the baby in 0.5s? The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?
  10. 10. Machines A machine is a device that makes doing work easier. It increases the force that you apply.
  11. 11. Machines cont. Input Force: force that is applied to the machine. = Fin Output Force: the force applied by the machine. = Fout Mechanical advantage is the ratio of the output force to the input force MA = Fout/Fin Efficiency % = Output work/ input work x 100
  12. 12. Machines cont. Friction causes energy of the output force to be lost as heat. This means Wout is smaller than Win How would lubricating a machine affect the output force exerted by the machine? Ideal MA is one without friction
  13. 13. MA and Efficiency Triangles Create a triangle for Mechanical Advantage Create a triangle for efficiency. Fout MA x Fin Fout Eff. x Fin x 100%
  14. 14. Calculating MA and Efficiency The mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N. Find the efficiency of a machine that does 800 J of work if the input work is 2,400 J.
  15. 15. Simple Machines 6 basic types Levers Pulleys Wheel and Axel Inclined Planes Screw Wedge View Simple Machines
  16. 16. Levers There are 3 types of Lever: First-Class: Fulcrum is between input & output force. Second-Class: Output force is between the fulcrum & the input force Third-Class the input force is between the fulcrum and the output force. View Examples
  17. 17. Pulleys  Fixed: Changes only the direction of YOUR force. (4N in gets 4N out)  Movable: the attached side of the rope supports half of the weight. Ex. For a 4N object attached side supports 2N while you only do 2N of work.  Block and Tackle: Made up of 2 fixed & 2 movable pulleys & weight is divided evenly among all of them. View Pulleys