How to calculate the torque from a free rotation object?

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Discussion Overview

The discussion revolves around calculating torque in a system where a rigid body, specifically a cylinder, is rotating freely about its z-axis. Participants explore the implications of constant angular velocity, the relationship between torque and net forces, and the effects of friction and resistance in practical scenarios, such as a coasting car.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how to calculate torque at the center of a rotating cylinder and whether the net force is zero despite rotation.
  • Another participant asserts that if angular velocity is constant, there is no net torque, suggesting that any torques present must cancel each other out.
  • A participant seeks clarification on how to determine equal and opposing torques if the net torque is zero.
  • Concerns are raised about the effects of friction and rolling resistance on a coasting car, with a suggestion that net torques on the tires and drive shaft would be zero if speed remains constant.
  • One participant expresses curiosity about the torque experienced when a finger is placed in the center of a spinning wheel, questioning the source of that torque.
  • Another participant draws an analogy to the forces experienced when trying to grab a passing train, discussing the factors that influence the force exerted on the finger.
  • A question is posed regarding how to calculate the force when two objects of differing speeds come into contact.

Areas of Agreement / Disagreement

Participants express differing views on the nature of torque in a rotating system, particularly regarding the implications of constant angular velocity and the effects of external forces. The discussion remains unresolved with multiple competing perspectives on the calculation and implications of torque.

Contextual Notes

Participants mention various factors that could influence torque and force calculations, including friction, rolling resistance, and the interaction of speeds between objects. However, specific mathematical steps or assumptions required for these calculations are not fully explored.

Who May Find This Useful

This discussion may be useful for individuals interested in the mechanics of rotational motion, torque calculations, and the effects of friction in dynamic systems.

keithlaw
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Hi everyone,

A question just pop up from my head.

Imagine a rigid body cylinder is rotating freely about its z-axis with a known angular velocity. Let's say the whole system is frictionless. How can I get the value of the torque at the center of the z-axis?

Is the net force on this free rotational body equals zero even though it is rotating?


Why I have a thought of it is because I want to know if the spinning wheel of a car will apply any torque to the drive shaft if the car is coasting.
 
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If angular velocity of an object is constant, then there is no net torque on that object. There could be equal and opposing torques on that object that cancel out, resuting in a net torque of zero.
 
rcgldr said:
If angular velocity of an object is constant, then there is no net torque on that object. There could be equal and opposing torques on that object that cancel out, resuting in a net torque of zero.

"There could be equal and opposing torques on that object that cancel out, resulting in a net torque of zero."
Could you please rephrase this sentence? If there is no net torque, how can I calculate the equal and opposing torques?
 
keithlaw said:
If there is no net torque, how can I calculate the equal and opposing torques?
If there is no net torque, then you'd have to know all of the actual torque values except for one of them in order to determing the value of that one remaining torque value.

In the case of a car coasting, normally the car would be slowing down due to friction in the drive train and rolling resistance. If the car was going slightly downhill with the end result that speed was constant while coasting, then the net torques on the tires and on the drive shaft would be zero.
 
rcgldr said:
If there is no net torque, then you'd have to know all of the actual torque values except for one of them in order to determing the value of that one remaining torque value.

In the case of a car coasting, normally the car would be slowing down due to friction in the drive train and rolling resistance. If the car was going slightly downhill with the end result that speed was constant while coasting, then the net torques on the tires and on the drive shaft would be zero.

"If the car was going slightly downhill with the end result that speed was constant while coasting, then the net torques on the tires and on the drive shaft would be zero."
Great! That is what I want to know. Thank you very much.

But if I put my finger into the center of the wheel, the wheel is going to twist my finger, right? Then there should be a torque, right? Then where does this torque come from?
 
keithlaw said:
But if I put my finger into the center of the wheel, the wheel is going to twist my finger, right? Then there should be a torque, right? Then where does this torque come from?

That's the same effect as if you tried to grab onto a passing train. It's going to put a force on you.

It comes from the fact that the speeds of the two bodies (wheel/finger or train/you) are unmatched, and if you put them in contact then they will try to macth.

How hard it actually twists you finger depends on many factors...how soft your finger is, how wide it is, how hard you are pressing it into the wheel, how fast it's spinning, etc. Essentially it's the same as trying to determine how hard the train will pull on you. There are many factors involved. If you play your cards right, you might even be able to jump onto it and survive. If you play your cards wrong, you could end up a stain on its windshield.
 
Lsos said:
That's the same effect as if you tried to grab onto a passing train. It's going to put a force on you.

It comes from the fact that the speeds of the two bodies (wheel/finger or train/you) are unmatched, and if you put them in contact then they will try to macth.

Thank you for your reply. :smile:
So, how can we calculate this force when two different in speed objects contact each other?
 
Last edited:

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