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Chapter 6 Energy Energy Universe is made up of matter and energy. Energy is the mover of matter. Energy has several forms: – Kinetic, Potential, Electrical, Heat, etc. Work Now instead of a force for how long in time we consider a force for how long in distance. Work = Force Distance W=Fd The unit for work is the Newton-meter which is also called a Joule. Questions: How much work is done when a weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters above the ground? How much work is done when a weight lifter pushes on a stationary wall with a force of 1000 Newtons for 15 seconds? Power Power is equal to the amount of work done per unit time. work done Power time interval The unit for power is the Joule/second which is also called a Watt. Light Bulbs and Appliances electrical energy used Power Rating time interval How much energy does a 100 Watt light bulb use in one hour? How about a 40 Watt light bulb? Which bulbs shines brighter? Mechanical Energy When work is done on an object, the object generally has acquired the ability to do work. This "ability to do work" is called energy and it has the same units as work….Joules. Two Types of Mechanical Energy – Potential Energy and Kinetic Energy Potential Energy The energy that is stored is called potential energy. Examples: – Rubber bands – Springs – Bows – Batteries – Gravity? Gravitational Potential Energy PE = Weight height PE = m g h Question: – How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground? Kinetic Energy Kinetic Energy is the energy of motion. Kinetic Energy = ½ mass speed2 1 2 KE mv 2 Question: How much kinetic energy does a 1kg mass have if it is moving at 10 meters/second? Work/Energy Relationship If you want to move something, you have to do work. The work done is equal to the change in kinetic energy. Work = DKE Example Question When the brakes of a car going 90 km/h are locked, how much farther will it skid than if the brakes lock at 30 km/h? • Answer: 9 times Conservation of Energy Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. Demos – Galileo's incline – Bowling ball pendulum – Loop the loop Example Problem A 100 kg mass is dropped from rest from a height of 1 meter. How much potential energy does it have when it is released? How much kinetic energy does it have just before it hits the ground? What is its speed just before impact? How much work could it do if it were to strike a nail before hitting the ground? 100 kg KE 12 mv 2 0 PE mgh (100kg)(9.8m / s 2 )(1m) 980J 1 meter 100 kg nail 100 kg KE 12 mv 2 980 Joules PE mgh 0 Joules Work Done Force Distace 980 Joules Machines - An Application of Energy Conservation If there is no mechanical energy losses then for a simple machine... work input = work output (F d)input = (F d)output Examples - levers and tire jacks Efficiency work done Efficiency energy used Useful energy becomes wasted energy with inefficiency. Heat is the graveyard of useful energy. Comparison of Kinetic Energy and Momentum Kinetic energy is a scalar quantity. Momentum is a vector quantity. Discuss rubber bullets as compared to lead bullets. Example Questions A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work? (a) 10 lb weight (b) 20 lb weight (c) same work for each lifting (d) not enough information is given to work the problem Two cars, A and B, travel as fast as they can to the top of a hill. If their masses are equal and they start at the same time, which one does the most work if A gets to the top first? (a) A (b) B (c) they do the same amount of work An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest? (a) 180 Joules (b) 2700 Joules (c) 36 Joules (d) 5 Joules (e) 180 N A 20 Newton weight is lifted 4 meters. The change in potential energy of the weight in Newton.meters is (a) 20 (b) 24 (c) 16 (d) 80 (e) 5 Your Mission: • Print out the study guide and define each term. • Read all of the “blue questions” and answers in Chapter 6. • Make sure that you have read this chapter. End of Chapter 6