advertisement

Chapter 7 Energy Introduction Universe is made up of matter and energy. Energy is the mover of matter. It has several forms. To understand this concept we will begin with a closely related physical concept. 1. WORK Now instead of a force for how long in time we consider a force for how long in distance. Work = Force x distance or W = F.d Units - Joules (J) or ft.lb BTU = 778 ft.lb (energy of one wooden kitchen match) Pushing on a wall and wall doesn’t move (no work done on the wall) F Fy Fx x W Fx x Video Clip Manpowered Machines 2. POWER Power = Work/time or P = W/t Watt Units - J/s = W 550 ft.lb/s = 1 hp 1 hp = 746 J/s = 746 W 1 BTU/hr = 0.293 W 100 W bulb = 0.1341 hp 250 hp engine = 186,450 W Chapter 7 Review 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 3. MECHANICAL ENERGY When work is done on an object, the object generally has acquired the ability to do work. This is called energy and it has the same units as work. Two Types of Mechanical Energy Potential Energy Kinetic Energy Potential Energy Energy of position or configuration Demo - Dart gun Other examples - Springs, bow, sling shot, chemical energy, and gravitational potential energy The latter is GPE = mgh The potential energy of an object depends on a reference position. It represents the work done against gravity to put the mass m in its position h above some reference position. It is an energy of position. Video Clips Incline Screw Kinetic Energy It is an energy of motion. KE mv 1 2 2 It is a square law. Total Work (work done by all forces acting on mass m) = DKE Work to Stop KE 1 2 0 mv mv 2 f 1 2 1 2 mv 2 i 2 i Fx x Fx x Note 1 2 m( 2vi ) 2 1 2 m4v 2 i x 2 1 4 ( mv 2 i ) Fx Work-Energy Theorem The net work done on an object is equal to the change in the kinetic energy of the object. Net Work = DKE Chapter 7 Review Question A 20 pound weight is lifted 4 feet. The change in potential energy of the weight in ft.lb is (a) 20 (b) 24 (c) 16 (d) 80 (e) 5 4. CONSERVATION OF ENERGY Galileo's inclines Demo - Bowling ball pendulum Demo - Loop the loop Video - Pole Vaulting Energy lost due to friction is actually not a loss; it is just a conversion. Energy cannot be created or destroyed. It may be transformed from one form into another, but the total amount of energy never changes. Energy Conservation in Satellite Motion (Next slide) Perigees Circle Ellipse Ellipse Apogees Energy is conserved along all of these paths. Video Clips Driving Nails Water Wheel Roller Coaster Condition for Conservation of Mechanical Energy No work can be done on the object by a nonconservative force. A nonconservative force is a force that converts mechanical energy into another form. Example: Friction No work is required to maintain circular motion at constant speed. E mc 2 Chapter 7 Review Question 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 5. Machines If no losses then work input = work output (F.d)input = (F.d)output Examples - levers, block and tackle, etc. Demo - Block and tackle Demo - Hydraulic lift F D D F = D D 6. EFFICIENCY Efficiency = work done/energy used Useful energy becomes wasted energy with inefficiency. Heat is the graveyard of useful energy. EER = energy efficiency ratio It is the output capacity(BTU/hr)/input energy(Watts) (Output capacity represents energy moved.) 7. COMPARISON OF KINETIC ENERGY AND MOMENTUM KE is a scalar and cannot be canceled. pi pf 0 Dp p f pi I This Thief is the absorbs impulse allapplied the kinetic to the energy. man. This is the impulse applied to the bullet. pi pf Dp p f pi - pi I ThiefThis does allfrom the kinetic energy isisnot the impulse previous slide. This theabsorb impulse applied to the man. in this latter example. This is the impulse applied to the bullet. Rubber bullets versus lead bullets Slow and fast football players with different masses. Consider head-on with one having twice the mass but half the speed of the other. Twice the mass at half the speed. Momentum can cancel. ( 2 m )v m( 2v ) 0 Kinetic energy is not a vector and cannot cancel out. The kinetic energy of the big slow person is 2 2 1 KE1 ( 2 m )v mv 2 The kinetic energy of the small fast person is 2 2 1 KE2 m( 2v ) 2 mv 2 Punch is the same but the energy delivered is not. 8. SOURCES OF ENERGY Except for nuclear and geothermal power, the source of practically all our energy is the sun. Nuclear power Geothermal power Solar power Wind power 9. ENERGY FOR LIFE Reading assignment