1. 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

2. 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 (1818-1889) known for his mechanical equivalent of heat experiments 11/22/2015APHY1012

3. 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

4. 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

5. 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

6. Work, Energy and Power   Kinetic Energy   The energy of motion   It is always positive: KE = ½ m v 2 11/22/2015APHY1016

7. 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 (1736-1819) improved the steam engine 11/22/2015APHY1017

8. 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

9. Work, Energy and Power 11/22/2015APHY1019

10.   Machines   They multiply forces, not energy !!!   Work input = Work output Assumes thermal losses are small   Examples: ramps, levers, pulleys 11/22/2015APHY10110