Conceptual Physics Study Notes & Questions: Energy (Chap. 8)

1. Conceptual Physics Study Notes & Questions: Energy (Chap. 8) • Work is done—that is, energy is used—whenever a forces is exerted over a distance (p164): W = Fd. What is the MKS unit of work (i.e. energy)? • How much work is done on a 10kg mass, lifting it 2 meters, at the surface of the earth (p165)? • Energy is the ability to do work (p166). Power is the rate at which energy is expended (i.e. the rate at which work is done). What is the MKS unit for power. • A non-active human expends about the same power as a 100 watt light bulb. • 1 food Calorie equals 4184 joules of energy. A 180 Calorie soft drink contains ~753,000 joules of chemical energy—enough energy to lift a 80kg person about 3000 feet. In comparison, the same amount of energy is used by the simple act of living for about 2 hours.) One pound of body fat contains 3500 Calories of chemical energy—enough energy to sustain human life for about 40 hours. • Energy falls into three basic categories: • energy bound up in rest mass (E = mc2), • kinetic energy (KE = ½ mv2), and • potential energy (PE)—energy stored in force fields—in its various forms: chemical energy, gravitational energy, mechanical energy (eg. a wound spring), nuclear energy, electrical energy, etc… • Kinetic energy, KE, is energy stored in momentum (p169), BUT it is not the same as momentum. Momentum is preserved in collisions, but kinetic energy usually is not (it is lost as heat). In a perfectly elastic collision, for example, two gas atoms colliding, KE is preserved. • KE and PE are constantly interchanging in day-to-day life (p172). But usually KE, in the form of thermal energy, is siphoned off whenever two objects interact.