Turning Force vs. Accelerating Force

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Discussion Overview

The discussion revolves around the effects of applying forces to a sphere or 3D object, particularly focusing on the differences between forces applied through the center of mass (COM) and those applied elsewhere. It explores concepts of linear acceleration and rotational motion, as well as the implications of applying forces at different points on the object.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that applying a force through the COM results in linear acceleration according to F=ma, while a force applied elsewhere creates a turning force described by T=F*d.
  • One participant suggests that Newton's second law applies regardless of whether the force is through the COM, indicating that the body will still accelerate.
  • There is a question about whether a force applied to the edge of an object has the same accelerating effect as one applied through the COM, with some participants expressing uncertainty about this point.
  • Another participant claims that applying a force at the edge does have the same effect, countering the initial doubts and referencing experimental validation in spacecraft.
  • Concerns are raised about the relationship between force application and the resulting power and energy, with one participant noting that forces causing rotation require more work and thus increase total energy more than linear forces alone.
  • A participant points out a related thread but another questions its relevance, emphasizing the difference between point masses and non-point mass objects in the context of force application.
  • A further inquiry is made regarding the placement of frictional forces on a free body diagram, particularly in relation to the object's tendency to spin.

Areas of Agreement / Disagreement

Participants express differing views on the effects of applying forces at various points on an object, particularly regarding the implications for acceleration and rotation. There is no consensus on whether forces applied at the edge have the same effect as those applied through the COM, and the discussion remains unresolved on several technical aspects.

Contextual Notes

The discussion involves complex interactions between linear and rotational dynamics, with participants referencing various principles of physics without fully resolving the implications of their claims. The nuances of how forces affect motion and energy in different scenarios are not completely addressed.

thepopasmurf
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If I have a sphere (or another 3D object) and I apply a force to it through the centre of gravity, it will accelerate according to F=ma
If I apply a force to the sphere which doesn't act through the centre of gravity, I apply a turning force according to T=F*d

How do you figure out how much the body as a whole accelerates when you apply a turning force to it?
 
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thepopasmurf said:
If I have a sphere (or another 3D object) and I apply a force to it through the centre of gravity, it will accelerate according to F=ma
If I apply a force to the sphere which doesn't act through the centre of gravity, I apply a turning force according to T=F*d

How do you figure out how much the body as a whole accelerates when you apply a turning force to it?
Simply apply Newton's second law and well as the rotational equation of motion. Newton's second law is still valid even if a the force is not applied through the COM of the body.
 
But does applying a force to the edge of an object have the same accelerating effect?
I imagine that a glancing blow won't move an object as much as a force acting through the COM
 
thepopasmurf said:
But does applying a force to the edge of an object have the same accelerating effect?
Yes, it does. It seems a bit counterintuitive, but that's how things work. This has been very well tested -- e.g., in spacecraft .
 
Last edited by a moderator:
That's correct.
 
thepopasmurf said:
That is counter-intuitive. So, on this diagram, all three equal magnitude forces will accelerate the block the same amount in the same direction? (And two will cause it to rotate?)http://img142.imageshack.us/img142/32/forcesre1.th.jpg
The forces are the same, but not the power. The forces that that cause rotation (angular acceleration) have to be applied at a faster rate of acceleration, at faster speed, and longer distance, so more work (force times distance) is done for the forces that cause rotation, which is why the power is higher, and the total energy, linear and angular, is increased more by the forces that also cause rotation (angular acceleration).
 
Last edited by a moderator:
How is that thread related at all? That thread is about particles: Point masses. This thread is about what happens to a non-point mass object subject to a force that does not pass through the object's center of mass.
 
  • #10
This question is related to my above question. Friction doesn't act through the center of mass, but then where on a free body diagram do you place the force which keeps the object (say a block) from spinning?
 

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