Transcript Lecture File 03

L3
Energy and Power
To discuss energy, we first discuss
the concept work done on an
object.
Work done is defined as the
product of a force exerted on an
object times the distance through
which it is exerted.
More specifically,
 we consider a force acting through a distance.
Work = Force x distance or W = F.d
Units - newtons x meters = joules (J), or
 pounds x feet (foot pounds, ft.lbs)
 BTU = 778 ft.lbs (energy of one wooden kitchen
match)
 Pushing on a wall and wall doesn’t move
(no work done on the wall)
Conversion: 1J= 0.738 ft.lb
But the concept of work done has
one other characteristic.
There is a directional aspect,
i.e.
the force involved is the part
(component) that is the direction
of the motion

F

Fy

Fx
x
W  Fx x
A 10 lb weight is lifted 5 ft. A 20 lb weight is
lifted 2.5 ft. Which lifting required the most
work?
(a) 10 lb weight
(b) 20 lb weight
(c) same work for each lifting
(d) not enough information is given to work the
problem
Two cars, A and B, travel as fast as they can to the
top of a hill. If their masses are equal and they
start at the same time, which one does the most
work if A gets to the top first?
(a) A
(b) B
(c) they do the same amount of work
3. 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
Potential Energy
Kinetic Energy
Potential Energy
Energy of position or configuration
Other examples - Springs, bow, sling shot,
chemical energy, and gravitational potential
energy
The latter is GPE = mgh (the force required to
lift at constant speed times the distance )
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.
Kinetic Energy
 If the body being lifted experiences an increase in speed, it
gains an energy of motion called kinetic energy.
KE  mv
1
2

2
It is a square law.
A 20 pound weight is lifted 4 feet. The
change in potential energy of the weight in
ft.lb is
(a) 20
(b) 24
(c) 16
(d) 80
(e) 5
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.
(from relativity, we find that mass is a form of
energy….)
CT 2.4.4
You are going skydiving. While you are free
falling what is happening to the energy in the
system?
a)
b)
c)
d)
e)
Potential energy is being transferred (or “transformed”) to
kinetic energy
Kinetic energy is being transferred (or “transformed”) to
Potential energy
The total energy of the system is decreasing
The total energy of the system is increasing
??
An object of mass 6 kg is traveling at a velocity of 30 m/s.
How much total work was required to obtain this velocity
starting from a position of rest?
(a) 180 Joules
(b) 2700 Joules
(c) 36 Joules
(d) 5 Joules
(e) 180 N
2. POWER
Power = Work/time or P = W/t
Units - J/s =
Watt
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
8. SOURCES OF ENERGY/POWER
Except for nuclear and geothermal power, the
source of practically all our energy is the sun.
Nuclear power
Geothermal power
Solar power (this includes biomass,
photovoltaics, solar thermal, etc.)
Wind power
We will discuss audio power in detail
later,
but keep in mind a few numbers:
A good audio amplifier is rated at about
100-200 W (input)
A trombone can produce about 6 W of
audio power.
MP3 players are in the tens of mW.
Note that these numbers describe the
sound source power.
We will discuss separately the loudness
of sound in terms of ” sound intensity”
in a unit to be defined later, called the
decibel (db).