Chapter 6 Work, Energy, and Power
Introduction Universe is made up of matter and energy. Energy is the mover of matter. It has several forms. To understand this concept we will begin with a closely related physical concept.
WORK For motion in a straight line the WORK done by a force is defined as the product of the component of the force in the direction of motion times the distance moved.
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l Work is a scalar quantity. l Work can be negative. l Work is the transfer of energy from one entity to another by way of the action of a force applied over a distance. The point of application of the force must move if work is to be done. l Pushing on a wall and wall doesn’t move (no work done on the wall)
The Units of Work l N. m {Joules (J)} or ft. lb l 1 erg = J. l 1 ft. lb = J. l 1 BTU = 778 ft. lb (energy of one wooden kitchen match)
ENERGY l Energy is a measure of the change imparted to a system??? l It can be mechanically transferred to an object when a force does work on that object. l Further, when an object does work, it gives up an amount of energy equal to the work it does.
MECHANICAL ENERGY l When work is done on an object, the object generally has acquired the ability to do work. l This is called energy and it has the same units as work. Two Types of Mechanical Energy Kinetic Energy Potential Energy
Kinetic Energy l It is the energy possessed by an object because of its motion. l It is a square law. Total Work (work done by all forces acting on mass m) = KE
Potential Energy l Energy of position or configuration l Demo – Dart Gun l Other examples - Springs, bow, sling shot, chemical energy, and gravitational potential energy l The latter is PE G = mgh
Gravitational Potential Energy l The potential energy of an object depends on a reference position. l It represents the work done against gravity to put the mass m in its position h above some reference position. l It is an energy of position.
Work to Stop KE 0 Note
The Work-Energy Theorem The net work done on an object is equal to the change in the kinetic energy of the object. Net Work = KE From text: when work is done on a point mass or a rigid body, and there is no change in PE, the energy imparted can only appear as KE. Insofar as a body is not totally rigid, however, energy can be transferred to its parts and the work done on it will not precisely equal its change in KE.
l Energy cannot be created or destroyed. l It may be transformed from one form into another, but the total amount of energy never changes. l Energy lost due to friction is actually not a loss; it is just a conversion. l Energy Conservation in Satellite Motion (Next slide) CONSERVATION OF ENERGY
Ellipse Parabola Hyperbola Energy is conserved along all of these paths. Perigees Apogees Circle
Condition for Conservation of Mechanical Energy l No work can be done on the object by a nonconservative force. l A nonconservative force is a force that converts mechanical energy into another form. l Example: Friction
l No work is required to maintain circular motion at constant speed.
POWER or
Units - J/s = W 550 ft. lb /s =1 hp 1 hp = 746 J/s = 746 W 1 BTU/hr = W 100 W bulb = hp 250 hp engine =186,450 W
The Kilowatt-Hour l The kilowatt-hour is a unit of energy. l If a force is doing work at a rate of 1 kilowatt (which is 1000J/s), then in 1hour it will do 1 kWh of work. l 1 kWh = 3.6 x 10 6 J = 3.6 MJ
Machines l If no losses then work input = work output (F. d) input = (F. d) output l Examples - levers, block and tackle, etc.
F D = F D D D
EFFICIENCY l Efficiency = work done/energy used l Useful energy becomes wasted energy with inefficiency. l Heat is the graveyard of useful energy. l EER = energy efficiency ratio It is the output capacity (BTU/hr)/input energy (Watts) (Output capacity represents energy moved.)