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Can the rotation of an object be viewed as the result of two perpendicular forces ('push' and 'pull') that have combined? When I spin a wheel there is a 'push force' at a right angle to the wheel's axis of rotation, but, without a some kind of a 'pull force' toward that axis there can be no rotation. Please help me understand the simplicity here. Thanks

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  • $\begingroup$ en.wikipedia.org/wiki/Centripetal_force $\endgroup$ Commented Nov 19 at 7:51
  • $\begingroup$ If you understand how parts of a rotating object accelerate, then by Newton’s Second Law you know the net force that must be acting on them. $\endgroup$ Commented Nov 19 at 8:13
  • $\begingroup$ This might help - Toppling of a cylinder on a block $\endgroup$ Commented Nov 19 at 15:12
  • $\begingroup$ Hi! Your rewritten question (v3) is different enough from the original (v1) that the existing answers don't make sense any more. That's a situation we try to avoid, so I'm going to revert the question to the first version. $\endgroup$ Commented Nov 19 at 17:24
  • $\begingroup$ @rob sorry about that. Can I post v3 then as a new question? $\endgroup$ Commented Nov 19 at 17:28

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Can the rotation of an object be viewed as the result of two perpendicular forces ('push' and 'pull') that have combined?

Rotation of an object can be the result of a combination of perpendicular forces, but it doesn't have to be.

Fig 1 below shows two equal and opposite parallel forces acting perpendicular to a rod. The torque produced by the two forces is called a "force couple", or simply a "couple." It produces angular acceleration (rotation) without translational acceleration, because the equal and opposite forces are a net force of zero.

Fig 2 below shows a single force acting perpendicular to the rod causing the same angular acceleration (rotation). However, since it constitutes a net force on the rod it also causes translational acceleration of its center of mass.

Hope this helps.

enter image description here

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You do not need two perpendicular forces to create rotation. A single off-center push or pull is enough. e.g. when you push on the edge of a door, you apply only a tangential force. You are not pulling toward the hinge, and the door still rotates.

The reason is that rotation comes from torque, not from a pair of perpendicular forces.

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  • $\begingroup$ Okay, but what about the earth's rotation around the sun or that of the moon around the earth? Does it also only require a single off centre push to keep going around in circles? $\endgroup$ Commented Nov 19 at 8:32
  • $\begingroup$ Ah... Did you mean the earth's orbit around the sun? in that case, the earth just moves in straight line and get pulled by sun's gravity, which make the earth's orbit around the sun. But if you mean the earth rotate around itself, we have to go back at the earth's formation from the rotating cloud of gas and according to conservation of angular momentum it causes earth to rotate until nowadays $\endgroup$ Commented Nov 19 at 9:00
  • $\begingroup$ You don't need to provide the force, but the centripetal force must still exist for rotation to occur - in this case, the material of the door itself provides the centripetal acceleration toward the hinge. You are not pulling toward the hinge, but the door is. After the initial push, there clearly is another force acting on the door, since it does not continue to move with constant velocity - it instead swings in a curved path around the hinge. An off-center push on a pile of sand, in contrast, won't make it rotate since there is no internal material centripetal force. $\endgroup$ Commented Nov 19 at 15:01
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Angular momentum is generated by torque, just like linear momentum is generated by force.

Angular momentum is rotation times moment of inertia, just like momentum is velocity times mass.

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