Not sure where to start

  • #1
[SOLVED] Not sure where to start....

10. A playground merry-go-round is mounted on a frictionless axle, so it can only rotate
in the horizontal plane. This object has a moment of inertia about the axle of I=50 kg m2,
and it has a diameter of 2.2 meters. Initially it is turning at a constant rate so that it
completes one revolution in 0.5 seconds. Standing next to it, you carefully place a block
of mass m= 5.0 kg on the rotating table, so that it sits a distance r=1.0 m from the center.
How long does it take to complete one rotation now?
1. 0.5 s
2. 0.55 s
3. 0.59 s
4. 0.63 s
5. 0.67 s
6. 0.71 s

all i have is that i found wi = 12.57 rad/s...but i don't know what to do after that?
 

Answers and Replies

  • #2
Shooting Star
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Can you think of any quantity which will remain conserved?
 
  • #4
Shooting Star
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No, because the mass distribution has changed after you put the mass on it.

If there's no external torque on a body, then the angular momentum remains the same. So, you should find the initial and final angular momenta.
 
  • #5
so initial and final angular momentum should equal since there is no external torque on the system right?
 
  • #6
Shooting Star
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You said it.
 
  • #8
I do have one more question...how do u recalculate the new moment of inertia?
 
  • #9
Shooting Star
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MI of the mass = mass*(dist from axis)^2
Final MI = MI of disk + MI of the mass
 
  • #10
okay, thanks
 

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