Transcript Chapter 6
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.
F
Fy
Fx
x
W Fx x ( F cos )x
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 Energy
Kinetic Energy
Potential Energy
Kinetic Energy
It is the energy possessed by an object
because of its motion.
KE mv
1
2
2
It is a square law.
Total Work (work done by all forces acting
on mass m) = DKE
Potential Energy
Energy of position or configuration
Demo – 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.
PEG mgh
Work to Stop KE
1
2
0
mv mv Fx x
2
f
1
2
2
i
1
2
mv Fx x
2
i
Note
1
2
m(2vi ) m4v 4( mv ) Fx
2
1
2
2
i
1
2
2
i
x
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 = DKE
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.
CONSERVATION OF ENERGY
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)
Perigees
Circle
Ellipse
Apogees
Energy is conserved along
all of these paths.
Ellipse
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
No work is required to maintain circular
motion at constant speed.
E mc
2
POWER
work doneby a force
Average power
tim e taken to do this work
or
P W
t
P Fd F v
t
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.
F
D
D=F
D
D
EFFICIENCY
Efficiency = work done/energy used
Useful energy becomes wasted energy with
inefficiency.
Heat is the graveyard of useful energy.
EER = energy efficiency ratio
It is the output capacity (BTU/hr)/input energy (Watts)
(Output capacity represents energy moved.)