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Kinetic Energy
Potential Energy
Kinetic Energy
You’ve probably heard of
kinetic energy
in previous courses using the following definition and
formula…
Any object that is moving has kinetic energy.
E
k
= ½
m
v
2
E
k
= kinetic energy (J)
m = mass (kg)
v = velocity (m/s)
We’re going to keep on using that basic formula, but we do need to clear up the definition a little bit.
What is “any object”?
•
“Any object” just refers to anything that we can measure as having a
mass
.
•
This covers everything from small subatomic particles like electrons all the way up to
galaxies.
When they say “moving” we need to ask “Moving relative to what?”
•
Right now you’re sitting motionless at a computer screen, so you have no kinetic
energy, right?
•
This is true relative to the reference frame of the room you’re in. Isn’t the earth
spinning on its axis? Isn’t the whole planet moving around the sun?
•
You need to make sure that you are always sure about what your measurements are
being taken in relation to.
•
Most of the time we measure stuff relative to the surface of the earth, so things
are easier, but be careful.
Example 1
: A pop can with a mass of 312g is sitting in the cup holder of my car as I drive down
Yellowhead at 68 km/h.
a)
Determine
how much kinetic energy it has relative to me in the car.
E
k
= ½ mv
2
But relative to me the pop can’s velocity is zero, so…
E
k
= 0 J
10/21/2005
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*Sign up*b)
Determine
how much kinetic energy it has relative to someone standing on the side of the
road.
E
k
= ½ mv
2
= ½ (0.312 kg) (19 m/s)
2
E
k
= 56 J
Also, be ready to manipulate this formula to solve for other variables…
Example 2
: Determine the velocity of a 150 kg cart if it has 3.60e4J of kinetic energy.
First, see if you can correctly solve the formula for “v”. This is one of the manipulations that

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