Ch 8 Energy Notes Concept Summary

Ch 8 Energy Notes Kinetic Energy If an object is moving, it has energy. (Be careful, the converse of this statement is not always true!) This energy is called kinetic energy - the energy of motion.

Kinetic Energy An object’s kinetic energy depends on: Ch 8 Energy Notes Kinetic Energy An object’s kinetic energy depends on: the object’s mass. Kinetic energy is directly proportional to mass. the object’s speed. Kinetic energy is directly proportional to the square of the object’s speed.

Ch 8 Energy Notes Kinetic Energy In symbols: 1 KE = mv2 2

Kinetic Energy Kinetic energy is a scalar quantity. Ch 8 Energy Notes Kinetic Energy Kinetic energy is a scalar quantity. Common units of kinetic energy: Joules An object with mass of 1 kg, moving at 1 m/s, has a kinetic energy of 0.5 Joule.

Ch 8 Energy Notes Work When the kinetic energy of an object changes, work has been done on the object. Units of work: Joules Work is a scalar quantity.

Work Work depends on: The amount of force applied to the object. Ch 8 Energy Notes Work Work depends on: The amount of force applied to the object. The distance that the object moves while the force is applied. The direction of the force with respect to the direction the object moves.

Ch 8 Energy Notes Work If the force on the object is in the direction the object moves, the work done is: W = Fx F x

Ch 8 Energy Notes Work If the direction of the force is opposite the direction the object moves, work is: W = -Fx F x

Ch 8 Energy Notes Force is NOT Work If the force is perpendicular to the direction the object moves, the work done is 0. If the object doesn’t move, the work done is 0. F W = 0 x

Work and Kinetic Energy Ch 8 Energy Notes Work and Kinetic Energy The work done on an object by the net force equals the object’s change in kinetic energy. Wnet = DKE

Ch 8 Energy Notes Potential Energy Sometimes work is not converted directly into kinetic energy. Instead it is “stored”, or “hidden”. Potential energy is stored energy or stored work.

Ch 8 Energy Notes Potential Energy Potential energy is energy that an object (system) has due to its position or arrangement.

Calculating Potential Energy Ch 8 Energy Notes Calculating Potential Energy To calculate the potential energy of a particular arrangement: Pick a position or arrangement that you want to call the “potential energy = 0” situation.

Calculating Potential Energy Ch 8 Energy Notes Calculating Potential Energy The potential energy of any other position or arrangement equals the negative of the work that the conservative force does in changing from the potential energy = 0 situation to that one. PE = - WorkF

Ch 8 Energy Notes Conservative Forces Energy or work is stored when a force does work “against” a force such as the gravitational force or a Hooke’s Law (spring) force. Forces that store or hide energy are called conservative forces.

Gravitational PE GPE = mgh Ch 8 Energy Notes Gravitational PE The gravitational potential energy of an object at height h equals the negative of the work that gravity does when the object is lifted from the PE = 0 position. GPE = mgh

Ch 8 Energy Notes Mechanical Energy Mechanical Energy = PE + KE

Conservation of Energy Ch 8 Energy Notes Conservation of Energy If no external forces act on a system, the total energy of the system will remain constant.

Power DWork Power = time W P t Power is the rate work is done. Ch 8 Energy Notes Power Power is the rate work is done. DWork Power = time W P t

Power Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt Ch 8 Energy Notes Power Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt Power is a scalar quantity.

(Simple) Machines A machine is a mechanical device used to do work. Ch 8 Energy Notes (Simple) Machines A machine is a mechanical device used to do work. Examples of simple machines: Inclined plane Lever pulley

Ch 8 Energy Notes (Simple) Machines A machine can never output more work (energy) than is put into it. At best, Workout = Workin Machine Workout Workin

Ch 8 Energy Notes Mechanical Advantage Machines can’t multiply work or energy, but they can multiply force. Mechanical advantage measures how much a machine multiplies force. Force machine exerts MA = Force you exert

Efficiency Useful work done Efficiency = x 100% Energy input Ch 8 Energy Notes Efficiency The efficiency of a machine tells how much of the energy (work) that goes into the machine actually does useful work. It is usually expressed as a percent. Useful work done Efficiency = x 100% Energy input

Ch 8 Energy Notes The End