Torque vs Centripetal Force: What's the Difference?

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Discussion Overview

The discussion centers on the differences between torque and centripetal force, particularly in the context of rotation and revolution. Participants explore the definitions and implications of these concepts within rotational dynamics and circular motion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that torque specifically causes rotation about an axis, while centripetal force causes revolution around another object.
  • Others challenge the clarity of the terms "cause rotation" and "cause revolution," questioning their definitions and implications in circular motion.
  • A participant suggests that for circular motion to occur, a centripetal force must be present, which can originate from various sources.
  • One participant describes a scenario involving a uniform-mass rod and the application of torque, indicating that the rod would rotate about its center of mass due to the applied torque.
  • Another participant argues that while the rod rotates, it also experiences translational motion, raising questions about the role of centripetal force in this context.
  • Some participants discuss the example of a ball on a string, noting that the tension in the string acts as centripetal force, causing the ball to revolve without applying torque on the ball itself.
  • There is a suggestion that the distinction between rotation and revolution may simply relate to the axis of rotation.
  • One participant mentions that any rotating rigid body can be viewed as a collection of point masses in circular translatory motion.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between torque and centripetal force, with no consensus reached on the definitions or implications of these concepts. The discussion remains unresolved regarding the distinctions between rotation and revolution.

Contextual Notes

Participants highlight the need for clarity in definitions and the implications of forces involved in circular motion, but do not resolve the underlying assumptions or conditions that affect their arguments.

PPERERA
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Torque causes a system to rotate about its axis while centripetal force causes a system to rotate about the axis of the separate system that exerted the centripetal force. So does this mean that torque specifically causes rotation and centripetal force causes revolution?
 
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while centripetal force causes a system to rotate about the axis of the separate system that exerted the centripetal force.
I don't think that statement makes sense. For a circular motion, there has to be a centripetal force, but that can come from anywhere.

What does "cause rotation" or "cause revolution" mean?
 
mfb said:
I don't think that statement makes sense. For a circular motion, there has to be a centripetal force, but that can come from anywhere.

What does "cause rotation" or "cause revolution" mean?

If it is true that centripetal force must be present for an object to undergo circular motion, then what I asked is not possible.

What I was trying to do was to see if there was a distinction between revolution (an object is in circular motion around another object that is not in circular motion) and rotation (an object in circular motion about an axis within itself) that could be explained by rotational dynamics.

The way that I was thinking about this was by imagining a uniform-mass rod in space (no forces are acting on it initially). A force is applied to the end of the rod. In this situation, I would expect the rod to being to rotate about the axis going through its center of mass as a result of the applied torque only. This would indicate that torque does cause rotation by itself.

So would the force cause the rod to rotate or would it only produce translational motion? And if the rod begins to rotate, is there a centripetal force involved?

Now clear your mind and imagine a ball attached to a string at a fixed position. A person swings this ball around, and the tension in the string acting as the centripetal force causes the ball to undergo circular motion about the stationary person (revolving around the person). Because there is no torque applied on the ball, the ball does not rotate about its center of mass. So the centripetal force specifically causes revolution but not rotation.

Thanks for the response.
 
PPERERA said:
What I was trying to do was to see if there was a distinction between revolution (an object is in circular motion around another object that is not in circular motion) and rotation (an object in circular motion about an axis within itself) that could be explained by rotational dynamics.
The difference is just the axis of rotation.

PPERERA said:
A force is applied to the end of the rod. In this situation, I would expect the rod to being to rotate about the axis going through its center of mass as a result of the applied torque only.
It would rotate around its center of mass, but it would also produce translational motion.
PPERERA said:
And if the rod begins to rotate, is there a centripetal force involved?
Internally in the rod, sure, as the outer parts do not move in a straight line.
PPERERA said:
Because there is no torque applied on the ball, the ball does not rotate about its center of mass
Oh, it would, at the same angular frequency as the rotation around the other end of the string, otherwise the string starts to roll up and that provides a torque.
 
PPERERA said:
A force is applied to the end of the rod.
The change in linear momentum of the rod equals the impulse (force x time) applied to the rod. The applied torque equals force x radius (distance from point of application to center of mass of the rod), and the change in angular momentum = torque x time.

PPERERA said:
Now clear your mind and imagine a ball attached to a string at a fixed position.
In order to get the ball moving or to change speed, a force in the direction of the velocity of the ball is required. The person does this by initially pulling on his / her end of the string, then twirling his / her end of the string in a circular path that is "ahead" of the path of the ball.
 
mfb said:
The difference is just the axis of rotation.
If think the difference the OP means is between rotation (changing orientation) and circular motion (translation along
a circular path without changing orientation)
 
PPERERA said:
What I was trying to do was to see if there was a distinction between revolution (an object is in circular motion around another object that is not in circular motion) and rotation (an object in circular motion about an axis within itself) that could be explained by rotational dynamics.
You can view any rotating rigid body as a collection point masses in circular translatory motion.
 

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