Angular Momentum, Inertia and Kinetic Energy??

  • #1
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Homework Statement



  1. A large wooden turntable in the shape of a flat disk has a radius of 1.50 m and a total mass of 100 kg. The turntable is initially rotating about its vertical axis through its centre with an angular velocity of 2.50 rad/s. From a very small height a 100 kg sand bag is dropped vertically onto the turntable at a point near the outer edge.

    a) Find the angular velocity of the turntable after the sand bag is dropped (assume that the bag can be treated as a point ).

    b) Calculate the kinetic energy of the system before and after the bag is dropped. Why are the kinetic energies not equal?


Homework Equations


I=1/2MR^2

The Attempt at a Solution


I've been stuck on this for a couple days now just playing around with it and haven't seem to get anywhere towards the answer.
 

Answers and Replies

  • #2
gneill
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Hi AndreewL, Welcome to Physics Forums.

You'll have to show some effort, showing what you've attempted during your "playing around with it". What avenues have you explored? What equations have you investigated?

Hint: This is effectively a collision type of problem involving angular motion rather than linear motion. What type of collision it? What is conserved in such collisions?
 
  • #3
robphy
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Clearly, there is a "initial" and "final" situation.
Can you try to identify what quantities are important in this problem?
What are those quantities (in terms of what is given) for the "initial" and "final" situations?
If there are unknowns, what laws of physics would be useful to relate the unknowns to the givens?
 
  • #4
gneill
Mentor
20,913
2,862
Clearly, there is a "initial" and "final" situation.
Can you try to identify what quantities are important in this problem?
What are those quantities (in terms of what is given) for the "initial" and "final" situations?
If there are unknowns, what laws of physics would be useful to relate the unknowns to the givens?
Please wait for the OP to respond to the initial hint before providing additional help. Forum rules require that an attempt at a solution or at least a solid demonstration of effort be shown before help can be given.
 
  • #5
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Clearly, there is a "initial" and "final" situation.
Can you try to identify what quantities are important in this problem?
What are those quantities (in terms of what is given) for the "initial" and "final" situations?
If there are unknowns, what laws of physics would be useful to relate the unknowns to the givens?
I worked out the initial moment of inertia to be 112.5, I'm just unsure on equations lining angular velocity and Inertia
 
  • #6
gneill
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20,913
2,862
I worked out the initial moment of inertia to be 112.5, I'm just unsure on equations lining angular velocity and Inertia
What are the corresponding equations for linear motion? The equations for linear and angular motion take the same form, just the corresponding variables are renamed accordingly.
 

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