Download PowerPoint Presentation - Slide 1

Work & Energy
"There is nothing new to be discovered in physics now,
All that remains is more and more precise
measurement.”
-Lord Kelvin
“Time is nature’s way of making certain that everything
doesn't happen all at once.”
-Woody Allen
Some Kinds of Energy
•
•
•
•
Kinetic Energy – the energy of moving objects
Heat, or thermal energy – of warm bodies.
Chemical Energy – of chemical reactions.
Gravitational Potential Energy – of a
gravitational field.
• Electromagnetic Energy – energy associated
with electric and magnetic forces.
• Mass Energy – all objects have energy by virtue
of their mass, the energy released in nuclear
explosions.
Kinetic Energy
This is “moving” energy
Marion Jones Sprints to Victory in the 200 meter.
Gravitational Potential Energy:
This is “stored”
energy
something has
when it is
above the
surface of the
Earth.
Example of
Potential Energy
Cheryl Haworth lifts 135.2 kg
a distance of 2 meters
Mass and Energy
One of Einstein’s many contributions was the
recognition that mass is simply another form of
energy. Mass and other forms of energy can be
interchanged, as can kinetic energy, potential
energy, heat, etc. The amount of energy
contained in an object with mass m is given by
the famous equation
E = mc2
where c is the speed of light. The speed of light
is equal to c = 3108 meters/second.
Conservation of Energy
• Energy is neither created nor destroyed but only
transformed from one form to another.
• In a closed system, the total amount of energy is
conserved. If we add up the amount of energy in a
closed system including all of the different forms, the
sum will not change with time.
• The total amount of energy never changes, it only moves
from place to place and from one form to another.
• Conservation of Energy applies not just to kinetic and
potential energy, as in the example, but to all kinds of
energy (heat, chemical, …)
Conservation of Energy
m = 50.9 kg
Work and Energy
Definition: Work is the energy added to an object through
the action of forces over a distance.
Work = Force x distance
W = Fxd
Joules = Newton x meters
James Prescott Joule (1818-1889)
Joule determined the
mechanical equivalent of heat by
measuring change in
temperature produced by the
friction of a paddlewheel
attached to a falling weight. He
made a series of measurements
and found that, on average, a
weight of 772 pounds falling
through a distance of one foot
would raise the temperature of
one pound of water by 1° F. This
corresponds to (772 ft lbs)(1.356
J/ft lb) = 59 453.6 Calories, or 1
cal = 4.15 Joules, in close
agreement with the current
accepted value of 1 calorie =
4.184 Joules.
The son of a
prosperous brewer
in North England,
Joule financed his
experiments
himself.
Power
The rate of doing work or
expending energy
P = Energy/Time
Rock climbers
gain a lot of
potential energy
but do so slowly,
at low power
Power
Training
Cyclists do work more
quickly than rock
climbers. They
expend more power.
Cadel Evans expends
about 10,000 Calories
in a 2 hour race. This
corresponds to a
power of roughly 6
kilowatts.
Racing: Tour de France
Training in Arizona
Niagara Falls
570,000 kg of water goes over the falls every
second.
The falls height is 55 meters.
Thus the potential energy of 1 kg of water is
E=mha=19.855 J = 539 J.
The total power is then
P=570,000539 Joules/sec=3.1108 Watts
or 310 Megawatts.
Climbing out of the Grand Canyon
How big a lunch is needed?
Energy from lunch =
Work to be done
W=
=
=
=
mgh
(62 kg)(9.82m/s2)(1500 m)
9.1x105 Joules
218 Calories
That is,
1 peach & a glass of milk
That’s it?
Nope – we’re inefficient
Humans work at roughly
15% efficiency. The
rest of the energy goes
into heat and non-work
productive movement.
So, to climb the Grand
Canyon, we need 1450
Cal !
Two club sandwiches, one
egg, 1 fruit and 1 glass
of milk.