chapter 6 work, energy, and power introduction universe is made up of matter and energy. energy is...
TRANSCRIPT
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.
Work is a scalar quantity. Work can be negative. 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.
Pushing on a wall and wall doesn’t move(no work done on the wall)
The Units of Work
N.m {Joules (J)} or ft.lb 1 erg = 10-7 J. 1 ft.lb = 1.355 J. 1 BTU = 778 ft.lb (energy of one
wooden kitchen match)
ENERGY
Energy is a measure of the change imparted to a system???
It can be mechanically transferred to an object when a force does work on that object.
Further, when an object does work, it gives up an amount of energy equal to the work it does.
MECHANICAL ENERGY
When work is done on an object, the object generally has acquired the ability to do work.
This is called energy and it has the same units as work.
Two Types of Mechanical EnergyKinetic Energy
Potential Energy
Kinetic Energy
It is the energy possessed by an object because of its motion.
2
21 mvKE
It is a square law. Total Work (work done by all forces acting
on mass m) = KE
Potential Energy
Energy of position or configuration
Demo – Dart GunDemo – Dart Gun
Other examples - Springs, bow, sling shot,
chemical energy, and gravitational potential
energy
The latter is PEG = mgh
Gravitational Potential Energy
The potential energy of an object depends on a reference position.
It represents the work done against gravity to put the mass m in its position h above some reference position.
It is an energy of position.
mghPEG
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.
Energy cannot be created or destroyed.
It may be transformed from one form into another,
but the total amount of energy never changes. Energy lost due to friction is actually not a loss; it
is just a conversion. Energy Conservation in Satellite Motion
(Next slide)
CONSERVATION OF ENERGY
Ellipse
Ellipse
ParabolaH
yperbola
Energy is conserved alongall of these paths.
Perigees
Apogees
Circle
Condition for Conservation of Mechanical Energy
No work can be done on the object by a
nonconservative force.
A nonconservative force is a force that
converts mechanical energy into another
form.
Example: Friction
Units - J/s = W 550 ft.lb /s = 1 hp
1 hp = 746 J/s = 746 W
1 BTU/hr = 0.293 W
100 W bulb = 0.1341 hp
250 hp engine = 186,450 W
The Kilowatt-Hour
The kilowatt-hour is a unit of energy. 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.
1 kWh = 3.6 x 106 J = 3.6 MJ
Machines
If no losses then
work input = work output
(F.d)input = (F.d)output
Examples - levers, block and tackle, etc.