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Torque here is not external, you can tell because the total angular momentum in the system is the sum of the angular momentum of the two disks. Therefore the two disks are what makes up the system, neither of them are an external object. They only exchange momentum between each other, as they have both applied torques to each other. It is the same concept as ...

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If the system is the two discs then the frictional forces apply internal torques which have a net value of zero - the internal torques are opposite in direction and equal in magnitude. If no external torques are applied then angular momentum is conserved.

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Parallel axis theorem is for the rotation of a body about a single axis that does not pass through the center of mass. But the earth's motion cannot be simply described in this way in the frame where the sun is at rest. It is more usually described as a combination of rotation and orbital movement, where each of these motions has different angular speeds. ...

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Actually, You can take arbitrary point, and may calculate the angular momentum, torque, and so on. Center of mass is just one of them. Although the physical variables may be seen different with respect to your choice of point, the law of the Physics will be same, and the total angular momentum will be conserved. If something goes different, double-check your ...

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The law of conservation of angular momentum states that when no external torque acts on an object, no change of angular momentum will occur. Yes there is friction between the discs,when they come into contact . Consider the resultant of the friction forces acting on the discs to be F. As shown above they are an action-reaction pair.They are internal forces. ...

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First of all I suggest you take a look on how to use MathJax for correct formatting of the question as it is a bit difficult to read. Now to the physics. Actually the torque of a force is defined as $$\vec{M}=\vec{r}\times \vec{F}$$ where $\vec{r}$ is the distance from a certain point to the point where the force is exerted. Work is defined as W=\int \...

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Torque on earth due to sun is 0 as gravitational force always acts towards center. In the formula of torque of torque which is $\tau = \ rF sin\theta$. Now $\theta$ becomes 0 as Force and r vector acts on same line (r is perpendicular distance from the force F)and hence overall torque becomes 0. As net torque is 0 about earth's center, therefore angular ...

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I am asking when does the Newton's second law for progressive movement ($F=am$) apply to rigid-bodies (with torque) and why is that the case? Does it only apply when the movement is without sliding or always etc.? No, $N2L$ applies regardless of whether the object is rolling without sliding, sliding without rolling or sliding only. In order to have rolling (...

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I will add to K.R.Park's answer by saying that just because angular momentum is conserved about any point doesn't mean it is a trivial task so to so. We pick the center of mass of the rod because this point will not be accelerating after the collision, so we can move to an inertial frame where the center of mass of the rod is at rest. However, if we follow ...

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