enter image description here A uniform disc has centre O, radius a and mass 2m. It is free to rotate in a vertical plane about a horizontal axis through O. A particle P of mass m is placed on the highest point of the rough edge of the disc and the system is slightly disturbed so that OP begins to rotate with the particle in contact with the edge. In the subsequent motion OP makes an angle θ with the upward vertical.

What will be the moment of inertia of the system about the centre of the disc. And how can we find the equation of rotational motion for the system.


closed as off-topic by Kyle Kanos, Danu, Qmechanic Apr 27 '15 at 19:24

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    $\begingroup$ Welcome to Physics.SE! Please note that this is not a homework help site. Please see this Meta post on asking homework questions and this Meta post for "check my work" problems. $\endgroup$ – 299792458 Apr 27 '15 at 18:29
  • $\begingroup$ Please look up "mass moment of inertia" for basic shapes and add them up using the "parallel axis theorem". $\endgroup$ – ja72 Apr 27 '15 at 23:27

For disc $I=(2m*r*r)/2$
for small body $I=m*r*r$
Hence net Moment of inertia is their sum.

  • $\begingroup$ Disc MMOI is $I=\frac{m}{2} r^2$ $\endgroup$ – ja72 Apr 27 '15 at 23:29
  • $\begingroup$ @ja72 Yes. I used that. $\endgroup$ – Ranko Apr 28 '15 at 6:35
  • $\begingroup$ No, you have $I=(2m r^2)/2 = m r^2$. $\endgroup$ – ja72 Apr 28 '15 at 18:02
  • $\begingroup$ I used 2m since the mass of the disc is given as 2m. $\endgroup$ – Ranko Apr 29 '15 at 9:38
  • $\begingroup$ Point taken.... $\endgroup$ – ja72 Apr 29 '15 at 12:29

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