Chapter 3 Energy

Work An applied force acting through a distance parallel to the force Units of work (and energy) = joule (J) Zero // distance, no work Displacement perpendicular to applied force, no work An applied force acting through a distance parallel to the force Units of work (and energy) = joule (J) Zero // distance, no work Displacement perpendicular to applied force, no work

Fig 3.3 Work Against Gravity

Power The rate at which work is done Units: watts (W) ; horsepower (hp) Example: Walking versus running upstairs The “power bill” - you actually pay for “work” (kWh), which is related to energy The rate at which work is done Units: watts (W) ; horsepower (hp) Example: Walking versus running upstairs The “power bill” - you actually pay for “work” (kWh), which is related to energy

Units of Power Horsepower (1 hp = 550 ft·lb/s) Watts (N·m/s or J/s) 1 hp = 746 W Horsepower (1 hp = 550 ft·lb/s) Watts (N·m/s or J/s) 1 hp = 746 W Fig 3.5

Convert 9.8 kW to hp

Motion, Position and Energy Work and energy are related Energy = ability to do work When work is done on something, a change occurs in its energy level Work and energy are related Energy = ability to do work When work is done on something, a change occurs in its energy level Next: Relationship between work and energy associated with position Relationship between work and energy of motion Next: Relationship between work and energy associated with position Relationship between work and energy of motion

Potential Energy (PE) Energy associated with “position” –“Potential” to then do work Gravitational Potential Energy (GPE) –Measuring h: need a reference position (or reference height) Work can “change” PE Potential Energy can “change” into Kinetic Energy Energy associated with “position” –“Potential” to then do work Gravitational Potential Energy (GPE) –Measuring h: need a reference position (or reference height) Work can “change” PE Potential Energy can “change” into Kinetic Energy

Kinetic Energy (KE) Energy associated with “motion” Results from work or change in potential energy Speed squared! Double the speed, KE increases by 4 Energy associated with “motion” Results from work or change in potential energy Speed squared! Double the speed, KE increases by 4

Energy Flow Energy is used to do work on an object. Work against five main groups of resistance: Inertia Gravity Friction Shape Usually combinations of four above What is the result? Energy is used to do work on an object. Work against five main groups of resistance: Inertia Gravity Friction Shape Usually combinations of four above What is the result?

Energy Conversion Any form of energy can be “converted” into another form Energy flows from one form to another in natural processes Example - pendulum Any form of energy can be “converted” into another form Energy flows from one form to another in natural processes Example - pendulum

Energy Conservation Energy is never created or destroyed Energy can be converted from one form to another but the total energy remains constant When energy is transformed –Working or heating is always involved 2 good examples of “Energy Conservation”: –A rock in “free-fall” –Swinging pendulum bob Energy is never created or destroyed Energy can be converted from one form to another but the total energy remains constant When energy is transformed –Working or heating is always involved 2 good examples of “Energy Conservation”: –A rock in “free-fall” –Swinging pendulum bob

Energy Forms Fig 3.17

Energy Forms Mechanical energy Kinetic plus potential energy (external) Chemical energy Energy involved in chemical reactions Radiant energy Electromagnetic energy Visible light = small part Electrical energy Charges, currents, etc. Nuclear energy Energy involving the nucleus and nuclear reactions