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in some books it says in dynamics torque must always be calculated about centre of mass?

Also is it true that a body when tries to rotate always rotates about centre of mass ie axis of rotation always pass through it. I think it may be because a body free to rotate has least I about O .

Also when a body rotates around some other object each particle of body is moving with different radius and hence has different velocities so how can this happen is body in translation or rotation and how is torque acting here eg consider planetary motion.

For last one : When any object say O (which is a rigid body having appreciable dimension wrt its orbit) is rotating about a fixed point C at centre of circular orbit then according to me the bady is not just translating but it is doing both translation and rotation simaltaneously as the points the particles are basically moving in circles with their centre on C but different particles have different radius of orbit as body has size hence different velocoties therefore not in pure translation so how is this possible ? How is a free body rotating about a external point not about centre of mass

Matt
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  • Also read related question: https://physics.stackexchange.com/a/81078/392. It answers your first question. – John Alexiou Apr 28 '17 at 04:10
  • Any on why always use center of mass? See this question – John Alexiou Apr 28 '17 at 04:13
  • And the second question ? It is no where related to the supposed duplicate – Matt Apr 28 '17 at 11:39
  • I believe this answer to you answered this. For a body to rotate not about the COM there has to be a net force applied to it. This can be external, or a reaction from a pin joint. – John Alexiou Apr 28 '17 at 11:45
  • No what kind of motion is of earth when it is revolving around the sun ? – Matt Apr 28 '17 at 11:46
  • The center of mass is under the influence of the force of gravity and hence the center of mass orbits around the sun. There is almost zero torque applied to the earth and so it maintains its spin axis. So overall the motion is that of an instanteneous screw motion as per linked answer. – John Alexiou Apr 28 '17 at 11:51
  • @ja72 I have edited my question please read the last portion I still cant understand the orbital motion. – Matt Apr 29 '17 at 00:29
  • A free body cannot rotate about any point other than the center of mass. For the case of a planet, the body is not "free" because it is under the influence of gravity from the Sun. This force changes the linear momentum of the planet. – John Alexiou Apr 29 '17 at 01:51
  • So only when there is a pure torque on a body ie centre of mass moves with constant velocity only then rotation is about centre of mass. Shouldn't it be in frame of reference of centre of mass always the body rotates about it whether centre of mass is accelerating or not. Ofcourse after considering pseudo forces. – Matt Apr 29 '17 at 02:51

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