Does Torque act on a disc rotating with constant angular velocity

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Discussion Overview

The discussion centers around the concept of torque acting on a disc rotating with constant angular velocity, particularly in the context of turbines. Participants explore the relationship between torque, angular acceleration, and power in rotating systems, raising questions about the conditions under which torque may be considered zero.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether torque is zero for a disc rotating with constant angular velocity, based on the relationship Torque = (moment of inertia) * (angular acceleration).
  • Another participant asserts that in real systems, such as turbine engines, torque is not zero due to aerodynamic loads and other factors, suggesting that torque is only zero in ideal conditions like a perfect vacuum.
  • A follow-up question asks if it would be appropriate to say that power produced by a turbine is zero under ideal conditions where torque is zero.
  • It is noted that while the net torque may be zero for constant angular velocity, there are still equal and opposite torques acting on the system, such as output torque and resistive torque, which allow for rotation.
  • One participant confirms the correctness of the power equation Power = (torque) * (angular velocity) in the context of turbines.

Areas of Agreement / Disagreement

Participants express differing views on whether torque can be considered zero for rotating bodies at constant angular velocity. While some acknowledge that net torque is zero, others emphasize the presence of opposing torques in practical systems, indicating that the discussion remains unresolved.

Contextual Notes

Participants highlight the importance of considering real-world factors such as friction and aerodynamic loads, which complicate the understanding of torque in rotating systems. The discussion also touches on idealized scenarios that may not reflect practical applications.

crazycyrus
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Hey Folks
I have rather a silly doubt i guess...
It goes like this

Consider a disc rotating about a fixed axis with constant angular velocity. Which means it has no angular acceleration ie it must be 0. Now since Torque=(moment of inertia)*(angular acceleration). hence the torque acting on the disc must be zero.
But i have come across situations where torque comes into the picture while calculating the power produced by say a Turbine(rotating with a constant angular velocity) . where Power=(torque)*(angular velocity)...?? Why is it so? isn't the torque 0 on circular bodies rotating with constant angular velocity?
 
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The torque in a real system will never be 0, especially in a turbine engine. The aerodynamic loads alone load the rotating components highly. The only time you would have the torque = 0 is in a perfect vacuum with no losses due to things like friction and windage.
 
Ok under those conditions(vaccum,no friction losses etc) will it be appopriate to say the power produced by a turbine rotating at constant angular velocity is 0 since torque is 0?
 
crazycyrus said:
isn't the torque 0 on circular bodies rotating with constant angular velocity?
If the angular velocity is constant, then the net torque is zero. But as FredGarvin says, you may need to apply a torque to overcome friction, etc.
 
If the angular acceleration of the disk is zero that means the net torque is zero.

In the case of a turbine rotating at constant angular velocity, you have equal and opposite torques acting on both sides of the central shaft. The first torque is the output torque produced by the turbine engine, and the second torque is the resistive torque of the prime mover (which might be a generator). Since they are equal and opposite, the turbine rotates at constant angular velocity.

The power equation you gave is correct.


crazycyrus said:
Hey Folks
I have rather a silly doubt i guess...
It goes like this

Consider a disc rotating about a fixed axis with constant angular velocity. Which means it has no angular acceleration ie it must be 0. Now since Torque=(moment of inertia)*(angular acceleration). hence the torque acting on the disc must be zero.
But i have come across situations where torque comes into the picture while calculating the power produced by say a Turbine(rotating with a constant angular velocity) . where Power=(torque)*(angular velocity)...?? Why is it so? isn't the torque 0 on circular bodies rotating with constant angular velocity?
 

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