Physics Help Forum Kinetic energy

 Periodic and Circular Motion Periodic and Circular Motion Physics Help Forum

 Jun 30th 2014, 09:03 PM #1 Junior Member   Join Date: Jun 2014 Posts: 19 Kinetic energy Hi there, I'm a bit confused on how to answer the below question: A roundabout in an amusement park is spinning. Eventually it comes to a stop. Does the law of conservation of angular momentum apply in this situation? This is what I'm thinking so far: No external forces are present, so angular momentum will be conserved. However kinetic energy is not conserved, hence the roundabout stops. But is this right? If so, what causes the kinetic energy to decrease?
Jun 30th 2014, 10:18 PM   #2

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 Originally Posted by Fizzle Hi there, I'm a bit confused on how to answer the below question: A roundabout in an amusement park is spinning. Eventually it comes to a stop. Does the law of conservation of angular momentum apply in this situation? This is what I'm thinking so far: No external forces are present, so angular momentum will be conserved. However kinetic energy is not conserved, hence the roundabout stops. But is this right? If so, what causes the kinetic energy to decrease?
The kinetic energy here is really rotational kinetic energy. The Newton's Law version of Newton's Second Law is sum tau = I omega. We see here that the angular speed is constant if there is no net external torque. As the angular speed of the farris is decreasing there must be an external torque on it.

Directly speaking then, since omega is not constant the angular momentum is not going to be conserved.

-Dan
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 Jul 1st 2014, 12:52 AM #3 Junior Member   Join Date: Jun 2014 Posts: 19 Thank you for your clear explanation- I get it now.

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