Where Does Frictionless Energy Go?

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Discussion Overview

The discussion revolves around the behavior of a spinning mass as its radius changes, specifically in a frictionless context. Participants explore concepts related to kinetic energy, angular momentum, and the implications of changing radius on speed and energy conservation.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes a device where a spinning mass transitions from a radius of 100 m to 1 m, questioning where the kinetic energy goes in a frictionless scenario.
  • Another participant asserts that reducing the radius will cause the object to spin faster due to conservation of angular momentum, suggesting that the work done results in increased rotational kinetic energy.
  • A different participant proposes that if the mass maintains a speed of 100 m/s at a radius of 100 m, it should also maintain that speed at a radius of 1 m, invoking conservation of momentum.
  • In response, another participant calculates the angular momentum and concludes that the speed must increase to maintain conservation of angular momentum, providing a formulaic explanation.
  • One participant claims that a moderator has accepted the idea that speed remains constant at 100 m/s regardless of radius, referencing a tetherball scenario.
  • Another participant challenges the previous claims, emphasizing the need to clarify whether the scenario involves actively reeling in the cable or simply allowing it to wrap around a pole, indicating that these scenarios yield different outcomes.

Areas of Agreement / Disagreement

Participants express disagreement regarding the conservation of speed and energy in the context of changing radius. Multiple competing views remain, with no consensus on the correct interpretation of the physical principles involved.

Contextual Notes

Participants do not fully agree on the definitions and conditions of the scenarios being discussed, leading to confusion about the application of conservation laws. There are also unresolved assumptions regarding the nature of the system (e.g., whether it is actively controlled or passively allowed to change).

aviator
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i have a device where a spinning mass switches from a radius of 100 m to a radius of 1m by means of gearing, going the mass of 1 kg at a speed of 100 m/s

lets consider it frictionless

in order to reduce the radius i have to aply a work, the energy to aply that work comes from the 100 m/s of the mass because there's no other energy in the system

so when the ball has a radius of 1 m it has a lower speed having lost kinetic energy

my question is if the device is frictionles where has this kinetic energy gone to?
 
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I`m not sure what you mean exactly, but when you reduce the radius of the spinning object it will start to spin faster (conservation of angular momentum), so it picks up rotational kinetic energy (equal to the work you performed to bring the mass closer to the axis of rotation).
 
a ball is geared to an axe in such way that the radius decreases constantly as it spins

would it be correct to say that because of conservation of momentum if when it has a radius of 100 m has a speed of 100m/s when it has a radius of 1 m still has a speed of 100 m/s?
 
No, since the initial angular momentum is rmv=(100)(1)(100) = 10.000 kg m^2/s, when the ball

spins with a radius of 1m, the angular momentum will be still be 10.000 kgm^2/s =(1)(1)(v). So v=10.000 m/s.

If you pull in the ball radially, the torque on the ball will be zero, so its angular momentum is conserved.
Linear momentum obviously is not, since the ball rotates (there's a centripetal force acting on it).
 
Last edited:
you are wrong its been accepted by a mod here that speed is kept at 100 m/s however radius, this case is called a tetherball
 
aviator said:
you are wrong its been accepted by a mod here that speed is kept at 100 m/s however radius, this case is called a tetherball
As usual, you have no idea what you are talking about. You must specify whether you are actively reeling in the cable (this is what Galileo assumed in his response to your vaguely worded question) or just letting it wrap around a pole (like the tetherball case). These are different scenarios with different answers.

Stick to one scenario and get it straight. Stop posting the same thing over and over.
 

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