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Energy
Unit 4
Energy is the most central concept
underlying all science.
Energy comes to us from the sun in the
form of sunlight, it is in the food we eat, & it
sustains life.
Persons, places, & things have energy, but
we observe only the effects of energy when
something is happening- only when energy
is being transformed from one form to
another.
Work= Force x distance
We do work when we lift a load against Earth’s
gravity. The heavier the load or the higher we
lift it, the more work we do. Two things enter
into every case where work is done: (1)the
application of the force, and (2) the
movement of something by that force.
If we lift two loads up one story, we do
twice as much work as we would in lifting
one load the same distance, because the
force needed to lift twice the weight is twice
as great.
Similarly, if we lift one load two stories
instead of one story, we do twice as much
work because the distance is twice as great.
Notice that the definition of work involves
both a force and a distance.
The weightlifter holding a barbell
weighing 1000N over his head does no
work on the barbell.
When the weightlifter raises the barbell
from the floor, he is doing work on it.
The resulting unit of work is the Newtonmeter, also called the joule.
Power is the rate at which work is done.
Power= work done
time interval
Twice the power means the engine
can do twice the work in the same
amount of time or the same amount
of work in half the time.
The unit of power is the joule per
second, or the watt.
The “something” that enables an object to
do work is energy. Energy is measured in
joules.
Mechanical energy is the energy due to the
position of something, or the movement of
something. Mechanical energy can be in the
form of either potential or kinetic energy.
Potential Energy
• Energy that is stored and held in readiness
• Work is required to elevate objects against
Earth’s gravity
• The potential energy due to elevated
positions is called gravitational potential
energy.
• Gravitational PE= weight x height
Kinetic Energy
• Energy of motion
• KE= 1/2mv2
• The kinetic energy of a moving object is
equal to the work required to bring it to that
speed from rest, or the work the object can
do while being brought to rest.
• Fd= 1/2mv2
The law of conservation of energy:
Energy cannot be created or
destroyed.
It can be transformed from one form
into another, but the total amount of
energy never changes.
When you consider any system in its
entirety, whether it is as simply as a
swinging pendulum or as complex as
an exploding galaxy, there is one
quantity that does not change:
energy. It may change form or it may
be transferred from one place to
another, but the total energy score
stays the same.
Machines
• A device used to multiply forces or simply
change the direction of forces
• Follows the concept of the conservation of
energy
Work input= work output
The ratio of output force to input force for a
machine is called the mechanical advantage.
No machine can put out more energy
than is put into it. No machine can
create energy. A machine can only
transfer energy from one place to
another or transform it from one
form to another.
An ideal machine would have 100%
efficiency. In practice, 100% efficiency
never happens. In any machine, some
energy is transformed into atomic or
molecular kinetic energy- making the
machine warmer. This wasted energy is
heat.
Efficiency= useful work output
total work input
Bud, a very large man of mass
130kg, stands on a pogo stick. How
much work is done as Bud
compresses the spring of the pogo
stick 0.50m?
After finishing her physics homework,
Sherita pulls her 50kg body out of
the living room chair and climbs up
the 5m high flight of stairs to her
bedroom. How much work does
Sherita do in ascending the stairs?
In the previous example, Sherita slowly
ascends the stairs, taking 10s to go from
bottom to top. The next evening, in a
rush to catch her favorite TV show, she
runs up the stairs in 3s.
On which night does Sherita do more
work?
On which night does Sherita generate
more power?
Legend has it that Isaac Newton
“discovered” gravity when an apple
fell from a tree and hit him on the
head. If a 0.2kg apple fell 7m before
hitting Newton, what was its change
in PE during the fall?
A greyhound at a race track can run
at a speed of 16m/s. What is the KE
of a 20kg greyhound as it crosses the
finish line?
In a wild shot, Bo flings a pool ball of
mass m off a 0.68m high pool table,
and the ball hits the floor with a
speed of 6m/s. How fast was the ball
moving when it left the pool table?
(Use the law of conservation of
energy.)
Frank, a San Francisco hot dog
vender, has fallen asleep on the job.
When an earthquake strikes, his
300kg hot dog cart rolls down Nob
Hill and reaches point A which is 50m
high at a speed of 8m/s. How fast is
the hot dog cart going at point B
which is 30m high when Frank finally
wakes up and starts to run after it?