Work, Energy and Power Introduction Energy is used to do work Mechanical – motion of objects and gravity Types of energy: solar, chemical, nuclear, petroleum, geothermal, hydroelectric, wind, … 11/22/2015APHY1011
Work, Energy and Power Work The product of displacement and the force acting in the direction of the displacement. Unit: joule ( J ) or Nm James Prescott Joule ( ) known for his mechanical equivalent of heat experiments 11/22/2015APHY1012
Work, Energy and Power Energy It is hard to measure, can be transferred between objects and can change form. Atoms, kicking a ball, wind to electric to light, etc. A scalar quantity, may be positive or negative and has units of joules like work. 11/22/2015APHY1013
Work, Energy and Power Potential Energy Gravitational potential energy (GPE) depends on the position of the object relative to an arbitrary point where the object’s GPE = 0. It can be positive or negative GPE = m g y Other forms: springs, rubber bands, food, … 11/22/2015APHY1014
Work, Energy and Power Work and Gravitational Potential Energy An object’s GPE will change if work is done against gravity or by gravity. Lifting a ball: work done against gravity or +GPE Drop a ball: work done by gravity or –GPE 11/22/2015APHY1015
Work, Energy and Power Kinetic Energy The energy of motion It is always positive: KE = ½ m v 2 11/22/2015APHY1016
Work, Energy and Power Power The rate at which work is done Unit: watts (W) or J/s Horsepower (hp) is still used for engines 1 hp = 746 W James Watt ( ) improved the steam engine 11/22/2015APHY1017
Work, Energy and Power Conservative and Non-conservative Forces Conservative: gravity Path independent Non-conservative: kinetic friction, air resistance Path dependent Example: designing roller coasters 11/22/2015APHY1018
Work, Energy and Power 11/22/2015APHY1019
Machines They multiply forces, not energy !!! Work input = Work output Assumes thermal losses are small Examples: ramps, levers, pulleys 11/22/2015APHY10110