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I have a question about UCM, as I am studying it in my Physics course. I understand that Non-UCM experiences a tangential acceleration component, which is the result of changing speed along its circular path.

My textbook gives an example of this in the form of a pendulum, which is given tangential acceleration when it rotates in a vertical plane due to the acceleration from gravity.

This makes sense to me. However, what doesn't make sense to me is a scenario under UCM, in which gravity does not affect tangential acceleration - because in UCM, tangential acceleration must equal zero. So is UCM just a fictitious scenario, or can circular motion really appear on Earth and experience a tangential acceleration of zero?


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3 Answers 3

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One example was already mentioned - a spinning wheel. Just a few more examples crucial for industry and everyday life: parts of car engines, such as shafts and axles, rotating parts of pumps and compressors, such as impellers and turbines, airplane engine turbines, rotors of electric motors - points of all these parts often rotate in vertical planes. Gyroscopes are another example, where rotation can take place in any plane. Let me note that balancing rotating parts of such equipment is often an extremely challenging task for high rotation speed.

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The free rotation of uniform bodies can proceed at any angle from the horizonatal because the gravitational forces balance out. Think of a spinning wheel.

Driven rotation can approach arbitrarily close to uniformity is the driving force far exceed the unbalanced gravitational forces.

An aside that started life as a comment but grew a bit out of hand.

"Horizontal" is an idea which requires a preferred direction in space. That happens on the surface of a planet because the interaction between the solid surface and the local gravity provides that preferred direction.

Most of the universe does not have a "horizontal" and experiences

  • orbital motion in all planes (though not all at the same frequency own), and for circular orbits this is uniform. The orbits of human deployed satellites are often nearly circular and run in many different planes.
  • rotational motion in all planes and for stable rotations this is uniform. The rotations of the planets of our solar system point in a bunch of funny directions.
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That's true, planetary orbits and the like are seemingly uniform. But how about on Earth itself, where we have a constant gravitational acceleration pointing towards its center? So in that context, my use of the word horizontal refers to motion perpendicular to the direction of the acceleration due to gravity. I hope that makes sense. –  capcom Sep 29 '12 at 20:50

example motion of earth around the sun.2=motion of moon around the earth.3=a car or bicycle along a circular track possesses circular motion

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protected by Qmechanic Jan 31 at 17:29

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