Download Energy and Momentum

Energy and Momentum
Energy
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Energy is the work required to do something
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Energy is a scalar quantity (it does not have a direction)
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Energy can take many forms, ie light energy, sound energy,
heat energy, mechanical energy, potential (stored) energy,
chemical energy and nuclear energy
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Energy transfers and transforms can be measured.
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Transform example: chemical->electric->mechanical
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The conservation of energy principle states that energy
cannot be created or destroyed.
Transferring energy
Listed below are different energy transfers:
Emission or absorption of EM or Nuclear radiation
●Heating and cooling of an object or substance as result of temp
difference.
●The action of a force on an object resulting in movement
●Student Task : In groups construct an energy transformation
flow chart for the light energy of a florescent light tube in your
class room.
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Woirk !!!!
Work and Kinetic Energy
The mechanical energy required to move an object a certain
distance is called work and is defined as:
Work  Force  displacement
Work  Fs
Kinetic Energy (KE) is the energy required to make an
object move.
So from the definition of work we can say that.....
• Maynard G. Krebs will always be best
remembered for his response whenever
anyone mentioned the subject of work. He
would instantaneously shudder, and let out a
plaintiff cry of "WORK!?!?"
Kinetic Energy
W
o
r
k
F
s -I
But, from Newton's Second law we know that

Fm
a
Substituting into I
W
o
r
k

(
m
a
)
s
We also know that
2 2
v

u
2
a
s
But initially
v2  2as
1 2
as  v
2
Multiply both sides by m
Kinetic Energy
Therefore, we can write work as:
1 2
Work  mv
2
Tadda!
Kinetic Energy Units
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In the SI (metric) system of units:
• m is expressed in kg (kilograms),
• v is expressed in ms-1 (meters per second),
• the energy is expressed in J (joules).
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Note that if the velocity of an object is doubled, its kinetic
energy is quadrupled.
Kinetic energy can also can be converted into other forms of
energy.
For example on mountain roads runaway lanes are often
provided for trucks that lose their brakes while going down
long hills. The runaway lane takes the truck back uphill and
allows the truck to coast to a stop as its kinetic energy is
converted to potential energy.
Questions
1. A truck with mass 5000kg is moving at 10m/s. What is the initial
kinetic energy of the truck?
2. Two cars of mass 1000kg traveling at 60km/hr collide head on
and lock together on impact (are stationary after impact).
i) what is the kinetic energy lost in the collision?
ii) where does it go?
3. A body of mass 50kg is traveling at 20m/s. It is then brought to
rest by a constant force in a distance of 5m.
Calculate: i) loss in KE of the body ii) Work done by the body
against the retarding force iii) the magnitude of the force
Transformation of Energy
Collisions: The energy lost in a collision is transformed into the
following energies
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Sound energy: some energy lost in the collision is transferred to the
air molecules, causing them to vibrate rapidly thus producing the
loud bang that we sometimes hear.
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Thermal energy: Heat is produced as the result of rapid movement
of particles within a substance. The KE of the object is transferred to
the particles within it, thus causing heat.
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Potential energy of deformation: potential energy stored in an
object as a result of changing its shape. An example of this would be
a spring.
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