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In the immortal words of Albert Einstein:
"Why doth dat ball move so?"
This magnificent web site will explain to you what rotational energy is and how we see it in everyday life.
First off, what is rotational energy?  Dozens of scientists have tried over the past few centuries to find the answer to this question.  And they came up with the response that rotational energy is using torque to move something in a circular fashion.
Real-Life Applications:
  1. 1. Tires of A Car
  2. 2. Soda Cans
  3. 3. Bowling Balls
  4. 4. Sports
  5. 5. Space Stations & Artificial Gravity
  6. 6. Kids Toys (Spinning Tops)
  7. 7. Water Wheels & Hydroelectric Power
Concepts
    The idea behind rotational energy is simple.  A common query is why water stays in the bottom of a bucket while you are swinging it around in a circle?  The answer has to do with rotational energy.  While spinning the bucket in a circle, centrifugal force acts upon the water, pushing it in the opposite direction of the bucket.  Also, a ball rolling down an inclined plane can have a constant velocity, but it will still have acceleration due to angular motion.  The ball is still changing direction with every turn, therefore, it still has acceleration.
Formulas
mgh=(1/2)mv^2+(1/2)Iw^2

Solid sphere: gh=(7/10)v^2

Hollow sphere: gh=(5/6)v^2

Solid Cylinder: gh=(3/4)v^2

Hollow Cylinder: gh=v^2

m=mass (kilograms)
g=gravity (9.8 meters per second^2) x sin q
h=height (meters)
r=radius (meters)
v=velocity (meters per second)
Mass cancels because it remains constant throughout the equation.

Sample Problems
1.    A bowling ball with a mass of 5 kilograms rolls down a hill at an angle of 35 degrees.  This object is a solid sphere.  If the hill is exactly 23 meters above sea level, solve for the velocity of the ball as it travels down the hill.
 

gh=(7/10)v^2
(9.8 x sin 35)(23)=(7/10) v^2
(5.621049076)(23)=(7/10) v^2
129.2841288=(7/10) v^2
(129.2841288)/(7/10)=v^2
184.6916125=v^2
(Take the square root)
v=13.59 meters/sec

2.    A plastic beach ball with a mass of .5 kilograms is rolling down a sand dune at angle of 41 degrees.  The sand dune is 7.5 meters above sea level.  This object is a hollow sphere.  Solve for the velocity of the beach ball.

gh=(5/6)v^2
(9.8 x sin 41)(7.5)=(5/6)v^2
(6.429378484)(7.5)=(5/6)v^2
48.22033863=(5/6)v^2
(48.22033863)/(5/6)=v^2
(Take the square root)
v=7.606865738 meters/sec

3.    Someone carelessly left a soda can on a freeway.  A massive earthquake occurs and the freeway splits in half approximately 33.2132154515142 meters from the soda can.  The can begins to roll down the freeway which is at an angle of 83 degrees.  The can starts at a height of exactly 252.132 meters.  The can is empty, which increases the fine to exactly $1000, however its' mass is exactly .35 kilograms.  The can is an empty cylinder, solve for the velocity of the soda can as it rolls down the freeway.

gh=v^2
(9.8 x sin 83)(252.132)=v^2
(9.726952286)(252.132)=v^2
2452.475934=v^2
(Take the square root)
v=49.52247908 meters/sec

This is fast enough, that if it struck a rescue worker in the head, it might cause serious injury.  Then the forensics team would be able to lift the finger prints from the can and charge the owner with both littering and assault.  So don't litter!

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