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Energy Chapter 8 Key Terms Work – The change in energy of an object W=Fxd Power – The time at which you complete work P=W/t Joule – The unit of work, Newton-meter Watt – The unit of power, joule per second Key Terms 2 Energy – The property of a system that enables it to do work Potential Energy – The stored energy that a body possesses because of its position PE = mgh Kinetic Energy – The energy of motion KE = ½ mv2 Work W=Fxd The amount of work done is only dependent upon the force of gravity on an object and the distance it is being raised. The objects of the same mass, but are being pulled at different angles up a ramp will have the same amount of work exerted upon them. Power P = W/t If equal loads are lifted to the same height, the forces and distances are equal, so the work is the same. What will change is the power, or rate at which work is done. Potential Energy PE = mgh The potential energy is only dependent to a height, h, above a given base level, and the weight, mg, of an object (Remember: weight is the force due to gravity) Kinetic Energy KE = ½ mv2 Heavy objects will have more kinetic energy than light ones Kinetic energy depends more on speed than mass The Conservation of Energy Energy cannot be created or destroyed; it may be transformed from one form to another, but the total amount of energy never changes. Conservation of Energy for Downhill Skiing Conservation of Energy in a Pendulum Conservation of Energy on a Roller Coaster Thought Question 3 A high diver has a potential energy of 10,000 J at the top of the diving platform. 1. What is her kinetic energy when she gets to the bottom? 2. What is her kinetic and potential energy when she is halfway through her dive? Machines A machine is a device used to multiply forces or simply change the direction of forces Every machine uses the concept of conservation of energy The lever is an example of a simple machine, one that changes the direction of force by doing work on one end Work input = work output Levers Pulleys Another type of lever that can multiply forces Mechanical Advantage Mechanical advantage = output force / input force Mechanical advantage = input distance/output distance Efficiency The ratio of useful energy output to total energy input, the percentage of the work input that is converted to work output Efficiency = useful work output/ total work input Efficiency = actual mechanical advantage/ Theoretical mechanical advantage Assignment Read Chapter 8 (pg. 103-117) Do Chapter 8 Assessment #31-47 (pg. 120-121) Appendix F #1-21 (pg. 672-673)