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Hope it made at least somewhat sense..

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- Thread starter aaaa202
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- #1

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Hope it made at least somewhat sense..

- #2

Matterwave

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However, in the case the net external force is applied at the center of mass (e.g. in the case of gravity), there is indeed no net torque. In that case, the rotations of the object are due to purely inertial motions (i.e. it's initial conditions contained rotations).

The motion is then a solution to the torque free Euler's equations.

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- #4

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So can someone explain to me how a rotation is caused in terms of internal and external forces. I know what a torque is and all that. What I find hard to grasp is the fact that nothing in nature dictates that an object MUST rotate. You can say: I know the center of mass will move in a straight line. Also you know that it is geometrically possible for an object to rotate about a fixed point. But what law in nature says that it MUST do so. As soon as it rotates, then yes, you can identify the rotating part of the motion, calculate the work done it and then find out the angular velocity, acceleration and so forth. But that is only after you have accepted the fact that a rotation exists as a phenomenon in nature and not just a solution to the equations of motion.

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If there is an external system of forces, then the body can move and rotate in just about any way. The center of mass might simplify the analysis in certain cases, but not always.

A common case, however, is motion under gravity, with an important subcase where gravity is uniform. This system of forces does not produce any net torque, so the spin of the body, if any, is unaffected by it. So it is common to say that gravity is applied at the center of mass only; but that's a simplification valid only for a uniform field of force.

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- #7

Matterwave

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So can someone explain to me how a rotation is caused in terms of internal and external forces. I know what a torque is and all that. What I find hard to grasp is the fact that nothing in nature dictates that an object MUST rotate. You can say: I know the center of mass will move in a straight line. Also you know that it is geometrically possible for an object to rotate about a fixed point. But what law in nature says that it MUST do so. As soon as it rotates, then yes, you can identify the rotating part of the motion, calculate the work done it and then find out the angular velocity, acceleration and so forth. But that is only after you have accepted the fact that a rotation exists as a phenomenon in nature and not just a solution to the equations of motion.

Basically conservation of angular momentum says something that is rotating initially must remain rotating unless there is external torques. Of course, one may then ask "why is there conservation of angular momentum?" To which physics can only say basically "because that is what experiment shows".

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The force of this interaction is quite large, but finite in real bodies. This sets a limit to the ability of a real body to rotate. Beyond a certain value of angular speed, a body cannot exist as a single body, because the force of interaction is not strong enough to bend the trajectories of its atoms to hold them together. No such limit exists for translation (the speed of light sets another limit, though).

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