How does kinetic energy change in circular motion?

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Discussion Overview

The discussion revolves around the behavior of kinetic energy (KE) for a body moving in circular motion, particularly focusing on how KE changes at different points in a vertical circular path. Participants explore the relationship between kinetic energy and gravitational potential energy (GPE) in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how kinetic energy behaves for a body in circular motion, initially assuming it remains constant due to constant speed.
  • Another participant states that in a horizontal circular motion, KE remains constant, while in vertical motion, KE varies inversely with gravitational potential energy, which is highest at the top and lowest at the bottom.
  • A third participant reiterates the classical kinetic energy equation and suggests that KE is maximum at the bottom of the circle and minimum at the top, with equal values at the sides.
  • One participant notes that the assumption of constant speed is crucial, indicating that if speed varies, KE will change accordingly, as seen in systems like a pendulum compared to a DVD in a vertical player.
  • Another participant emphasizes that KE cannot change if speed remains constant, pointing out the need for clarity in the original question regarding the conditions of motion.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which kinetic energy changes in circular motion, particularly regarding the influence of gravitational potential energy and the constancy of speed. No consensus is reached on the implications of these conditions.

Contextual Notes

There are assumptions about the nature of the circular motion (horizontal vs. vertical) and the influence of gravitational potential energy that remain unresolved. The discussion also highlights the importance of specifying conditions clearly when discussing kinetic energy in circular motion.

Tangeton
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Could someone explain to me how the kinetic energy of a body moving in a circle is like? I got tempted it stays constant as the speed is constant but that turned out to be false.

I am interested in places where the body is at top, side and bottom and so on. So I'd like to also know what the KE will be at those points...

Thank you
 
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If the circle is in a horizontal plane then the KE will not change. If the circle is in a vertical plane then the gravitational potential at the top will be 2mgr (r is the radius) and zero at the bottom. The KE will vary in the opposite way - according to the gpe in any position, giving constant total energy throughout the rotation.
 
Tangeton said:
Could someone explain to me how the kinetic energy of a body moving in a circle is like? I got tempted it stays constant as the speed is constant but that turned out to be false.

I am interested in places where the body is at top, side and bottom and so on. So I'd like to also know what the KE will be at those points...

Thank you

Kinetic energy classically follows the following equation:
$$
KE = \frac{1}{2}mv^2
$$
Where m is mass, and v is velocity. Assuming it's vertical in a uniform gravitational field, the KE will be maximum at the bottom, and minimum at the top. At the left and right, it will be equal.
 
Cool thanks for both answers.
 
sophiecentaur said:
If the circle is in a vertical plane then the gravitational potential at the top will be 2mgr (r is the radius) and zero at the bottom. The KE will vary in the opposite way - according to the gpe in any position, giving constant total energy throughout the rotation.

That assumes the object isn't being forced to rotate at a constant speed. For example a pendulum trades off PE vs KE as it rotates but a DVD in a vertical DVD player doesn't.
 
True. Except that, by definition, the KE cannot change if the speed remains the same. But the OP didn't specify clearly enough - this will happen if one doesn't know the subject well. A common PF problem.
 
Last edited:

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