2. Chapter 8 Work, Power, Energy and Machines

3. zWork: applying a force in the direction of motion zWork=Force x distance

4. zWork is measured in joules (J). 1 joule is equal to a force of 1 N exerted over a distance of 1 m.

5. Work = Force || x Distance In this case, the distance is the magnitude of the displacement. Only the component of force parallel to the displacement does work

6. x

7. Power zPower is equal to the amount of work done per unit time. The unit for power is the Joule/second which is also called a Watt.

8. zmeasured in watts (W) zOne watt of power is expended when one joule of work is done in one second. Power

9. What is horsepower? z 1 horsepower = 746 Watts z A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute.

10. More on horsepower... z1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F). zOne BTU is equal to 1,055 joules, or 252 gram- calories or 0.252 food Calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100-percent efficient.Calories

11. Energy…the BIG picture zThe Universe is made up of matter and energy. zEnergy is the “mover” of matter. zThere are many forms of energy. zConversions from one form of energy to another continually occur. zEnergy cannot be created or destroyed.

12. zEnergy possessed by an object due to its motion or its stored energy of position. zIt can either be potential energy or kinetic energy. zAll forms of energy are measured in joules (J). Mechanical Energy

13. z(PE) stored chemical energy or energy of position. zTypes: elastic, gravitational and chemical. Potential Energy

14. zGravitational Potential Energy = mass x gravity x height zE PE = mgh

15. z(KE) energy of motion. Kinetic energy depends on both mass and velocity Kinetic Energy

16. zKinetic energy = ½ mass x velocity ² zE k = 1/2mv 2

17. Mechanical Energy Conversions …total energy is constant

18. Calculate speed at positions B,C and D.

19. More Energy Transfer z High speed winds are used to do work on the blades of a turbine at the so-called wind farm. z Mech. Energy from the air gives the air particles the ability to apply a force to the blades. z As the blades spin, their energy is subsequently converted into electrical energy (a non- mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.

20. Energy Transfer... zThis diagram shows that the boys potential energy is changing. Explain what is happening, in other words how is the energy changing?

21. zEnergy cannot be created or destroyed. zEnergy can be transformed from one form to another, but the total amount of energy never changes. Law of Conservation of Energy

22. zMachine: a device used to multiply forces or change the direction of forces. zSimple Machine: a machine that has only one motion. Two basic simple machines are levers and pulleys.

23. zA bar that freely pivots around a fixed point called the fulcrum. A lever multiplies a force Lever

24. Classes of Levers 1st Class 3rd Class 2nd Class

26. Your Body as a Lever... 1st Class 2nd Class 3rd Class

27. zPulley: grooved wheel with a rope running through it. It changes the direction of the force.

28. Fixed Pulley zA pulley in which the wheel does not move. zchange the direction of the effort force. It does not increase the size of the effort force. yThe effort force is equal to the resistance force in a fixed pulley; therefore, the mechanical advantage (MA) of a fixed pulley is equal to 1.

29. Movable Pulley  A moveable pulley does not change the direction of the effort force but does increase the size of the force.  When the rope is pulled, the pulley and the load come up.  The mechanical advantage (MA) of a moveable pulley may be determined by counting the number of ropes that lift the load.

30. Pulley System z The pulleys are used to increase the mechanical advantage of the system. z A pulley system's mechanical advantage (MA) is equal to the number of supporting ropes.

31. Determine the MA of the following pulleys:

32. Other Simple Machines Wheel & Axle Screw Wedge

33. zWork input = work output assuming that there is no friction. zMechanical Advantage: (MA) the ratio of output force to input force

34. zAn ideal machine would be 100% efficient. zWork input would equal work output. zThis does not exist…due to friction, work output cannot equal work input. In an ideal situation…

35. zEfficiency: The ratio of useful work output to total work input. zE = (Work output / Work input ) 100 zor zE = actual MA / ideal MA