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Homework: p. 82 problems 10,
11, 14 & 15
• Describe work and
energy.
• Calculate potential
energy.
• Calculate kinetic energy.
• Apply the law of
conservation of energy
to explain the motion of
an object acted on by
gravity.
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energy
joule
work
potential energy
kinetic energy
law of conservation of
energy
• The ability to do work or cause change.
• What is work? Work is the energy used to move
something. We measure it in Joules.
• Energy has a number of different forms, all of which
measure the ability of an object to do work on or
change another object.
• In other words, there are different ways that an object can
possess energy.
What are the different forms
of Energy?
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Kinetic Energy
Mechanical
Potential Energy
Energy
Thermal, or heat energy
Chemical Energy
Electrical Energy
Electrochemical Energy
Electromagnetic Energy (light)
Sound Energy
Nuclear Energy
Define Potential Energy
Potential energy is
energy due to
position or stored
energy. Potential
energy is also
called gravitational
potential energy.
Define Kinetic Energy
Kinetic energy is
the energy of
motion. Kinetic
energy is measured
by how much is
work done to put
an object in motion
or to rest.
• By stretching a rubber
band, you give it
potential energy. A
vase on a shelf has
stored potential energy.
A football being held
by a quarterback has
potential energy until it
is thrown and it turns
into kinetic energy.
• A basketball player has
kinetic energy. The
movements that she
does show the energy
that is being displayed
while she is moving.
When you are running,
walking, or jumping,
your body is exhibiting
kinetic energy.
A 2 kg rock is at the edge of a cliff 20 meters above a lake. It
becomes loose and falls toward the water below. Calculate its potential
and kinetic energy when it is at the top and when it is halfway down. Its
speed is 14 m/sec at the halfway point.
You can assume the initial speed is 0 m/sec because the rock starts from rest.
E p= mgh and Ek = ½ mv2
Potential energy at the top: m = 2 kg, g = 9.8 N/kg, and h = 20 m
Ep=(2 kg)(9.8 N/kg)(20 m) = 392 J
Potential energy halfway down: m= 2 kg, g = 9.8 N/kg, and h = 10 m
Ep=(2 kg)(9.8 N/kg)(10 m) = 196 J
Kinetic energy at the top: m = 2 kg and v = 0 m/sec
Ek=(1/2)(2 kg)(02)=0 J
Kinetic energy halfway down: m= 2 kg and v = 14 m/sec
Ek=(1/2)(2 kg)(14 m/sec)2 = 196 J
a. Calculate the potential • b. Calculate the kinetic
energy of a 4 kilogram
energy of a 4 kilogram
cat crouched 3 meters
cat running at 5 m/sec.
off the ground.
• Answer: 50 J
Answer: 117.6 J