Document 6523588

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What is Energy?
Energy is the ability to cause change.
Forms of Energy: Energy is found in
different forms, such as light, heat, sound
and motion.
There are many forms of energy, but they
can all be put into two categories: kinetic
and potential.
Potential Energy
An object can store energy as the result of
its position. This stored energy of position is
referred to as potential energy.
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Gravitational Potential Energy
Gravitational potential energy is the energy
stored in an object as the result of
gravitational attraction of the Earth for the
object.
There is a direct relation between
gravitational potential energy and the mass of
an object.
More massive objects have greater
gravitational potential energy. There is also a
direct relation between gravitational
potential energy and the height of an object.
The higher that an object is elevated, the
greater the gravitational potential energy is.
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These relationships are expressed by the
following equation:
Gravitational potential Energy= force of gravity* Height
GPE = m *g * h
m= mass of the object, h : height of the object
and g: acceleration due gravity (9.8 m/s2 on
Earth).
Why is g= 9.8?
If the velocity and
time for a freefalling object being
dropped from a
position of rest
were tabulated, then
one would note the
following pattern.
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Calculate acceleration for each time interval.
Time (s)
Velocity (m/s)
Acceleration
(m/s2)
0
0
1
-9.8
-9.8
2
-19.6
-9.8
3
-29.4
-9.8
4
-39.2
-9.8
5
-49
0
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Questions:
1-A ball of mass 1 kg is thrown up to a height
of 12 meters. Calculate the increase in its
potential energy.
Solution:
GEP = mgh
GEP = (1)(9.8)(12)
GEP = 117.6 J
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2-How far above the surface of the earth
would the same ball (1 kg) have to be thrown
for its potential energy to increase to 1 MJ?
Solution:
GPE = mgh
106 = (1)(9.8) h
h = 106 / 9.8
h = 102 040.82 m
Kinetic energy is the energy of motion. An
object which has motion - whether it is
vertical or horizontal motion - has kinetic
energy. There are many forms of kinetic
energy - vibration (the energy due to
vibration motion), rotational (the energy due
to rotational motion), and translational (the
energy due to motion from one location to
another). To keep matters simple, we will
focus upon translational kinetic energy. The
amount of translational kinetic energy which
an object has depends upon two variables:
the mass (m) of the object and the speed (v)
of the object.
The following equation is used to represent the
kinetic energy
where m = mass of object; v = speed of object
 The unit of energy is Jules
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This equation reveals that the kinetic energy
of an object is directly proportional to the
square of its speed.
Questions:
1. Determine the kinetic energy of a 625-kg
roller coaster car that is moving with a
speed of 18.3 m/s.
KE = 0.5*m*v2
KE = (0.5) * (625 kg) * (18.3 m/s)2
KE = 1.05 x105 Joules
2. If the roller coaster car in the above
problem were moving with twice the speed,
then what would be its new kinetic energy?
If the speed is doubled, then the KE is
quadrupled. Thus, KE = 4 * (1.04653 x 105 J)
= 4.19 x 105 Joules.
3. Missy Diwater, the former platform diver
for the Ringling Brother's Circus, had a
kinetic energy of 12000 J just prior to
hitting the bucket of water. If Missy's mass
is 40 kg, then what is her speed?
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KE = 0.5*m*v2
12 000 J = (0.5) * (40 kg) * v2
300 J = (0.5) * v2
600 J = v2
v = 24.5 m/s
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4. A 900-kg compact car moving at 60 mi/hr
has approximately 320 000 Joules of kinetic
energy. Estimate its new kinetic energy if it
is moving at 30 mi/hr.
KE = 80 000 J
The KE is directly related to the square of
the speed. If the speed is reduced by a
factor of 2 (as in from 60 mi/hr to 30
mi/hr) then the KE will be reduced by a
factor of 4. Thus, the new KE is (320 000
J)/4 or 80 000 J.
The Law of Conservation of Energy
To scientists, conservation of energy is not
saving energy. The law of conservation of
energy says that energy is neither created
nor destroyed. When we use energy, it
doesn’t disappear. We change it from one
form of energy into another.
Example 2:
Oil burns to make heat -->
Heat boils water -->
Water turns to steam -->
Steam pressure turns a turbine -->
Turbine turns an electric generator -->
Generator produces electricity -->
Electricity powers light bulbs -->
Light bulbs give off light and heat
The mass of the ball = 10kg
The height, h = 0.2m
The acceleration due to gravity, g = 9.8 m/s^2
What is the potential energy of the ball at
position 1?
PE = 19.6J (J = Joules, unit of energy)
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Position 1 of the ball is where the Potential Energy
(PE) =19.6, while the Kinetic Energy (KE) = 0
As ball 1 is approaching position 2 ,PE is decreasing
while the KE is increasing
At exactly halfway between position 1&2 PE=KE
The Position 2 is where the Kinetic Energy is at its
maximum while the Potential Energy = 0. At this
point, theoretically, all the PE has transformed into
KE Therefore now the KE = 19.6J while the PE = 0.
The position 3 of the ball is where the Potential
Energy (PE) is once again at its maximum and the
Kinetic Energy (KE) = 0.
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Example 3: How much kinetic energy a 2 Kg
ball will have when it bounces on the floor
after being thrown to a height of 4 m above
the floor. (ignore friction)
KE Bottom =GPE top = mgh = (2) (9.8) (4)= 78 J