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Kinetic and Potential
Energy
UCR MESA
What is Energy?
What does it mean if you
have a lot of energy?
For students it means you
can run around, lift weights
and scream.
If you have energy you can
do things.
http://www.nv.doe.gov/news&pubs/photos&films/atm.htm
What is Energy?
Energy is the ability to do work
This is similar to our every day definition of
energy. You can’t run up the stairs (work)
without energy!
We have lots of types of energy. Can you
name a few?
Energy comes in many different
forms
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Kinetic Energy
Gravitational potential energy
Thermal Energy (heat)
Nuclear Energy (nuclear power, atomic bomb)
Electrical Energy (electricity)
Chemical Energy (food, fire, …)
We are going to look at the first two types of energy
Kinetic Energy
Kinetic energy is the energy of
motion
KE = ½ mv^2
Units: Joules (J)
m = mass
v = velocity
Let’s Practice
What is the kinetic energy of
this car when it is traveling
38.0 m/s?
What other information do
you need?
KE = ½ m v^2
= ½ (1550)(38.0)^2
= 1119100 J
= 1.11 x 10^6 J
Car’s mass = 1550kg
Potential Energy
Gravitational Potential Energy is
energy due to an objects
height above the ground
PE = mgh
m = mass
g = acceleration due to gravity
h = height
http://gaaf.com/pictures/200406_utah/image009.htm
Let’s Practice
A rock has a mass of 8.40 x 10^4
kg. The center of mass is 29.0 m
above the ground. How much
energy does it have?
PE = mgh
Identify the variables
m= 8.40 x 10^4 kg
g= 9.81 m/s^2
h= 29.0 m
PE = (8.4X10^4)(9.81)(29)
= 2.39 x 10^7 J
http://gaaf.com/pictures/200406_utah/image009.htm
What’s so important about PE and KE?
We call the sum of PE and KE mechanical
energy.
ME = KE + PE
Mechanical energy is important because it is
conserved (as long as there are no non
conservative forces, like friction)
Therefore, if one goes down, the other goes up by
the same amount.
Conceptual understanding
A penny is dropped off the Eiffel
tower (ignore air resistance). As
it falls, what happens to it’s
potential energy? What happens
to it’s kinetic energy?
As it falls, its velocity goes up, so its
kinetic energy goes up. It also
looses height so its potential
energy goes down.
However, mechanical energy stays
the same ME = KE + PE
From the California Standards Test
Copyright © 2004 California Department of Education.
From the California Standards Test
Copyright © 2004 California Department of Education.
From the California Standards Test
Copyright © 2004 California Department of Education.