Work done when point of application of force remains fixed

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Discussion Overview

The discussion revolves around the concept of work done by a force when the point of application remains fixed, exploring qualitative analyses and examples. Participants engage with theoretical implications and practical examples, questioning the conditions under which work is considered to be done.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions the validity of a statement claiming that work done by a force is zero if the point of application remains fixed, seeking clarification and examples.
  • Another participant suggests that the question may stem from a misunderstanding of how forces are applied and the definition of the system.
  • A participant provides an example of a person jumping, arguing that while the point of application remains fixed, internal energy is released, complicating the assessment of work done.
  • Concerns are raised about the wording of the question, particularly regarding what "fixed" means in different contexts, such as relative to the object or the surface.
  • One participant proposes a scenario involving a wheel on a conveyor belt, suggesting that since the point of application remains fixed, the work done is zero, prompting further discussion.
  • Another participant counters this by asserting that work is done on the wheel because the material at the point of application is moving, despite the point itself being fixed.
  • Participants discuss the distinction between the point of contact and the material at that point, highlighting how different reference frames can affect the interpretation of motion and work.
  • Clarifications are made regarding the terms "pseudowork" and "center of mass work," with participants refining their understanding of these concepts.

Areas of Agreement / Disagreement

Participants express differing views on whether work is done when the point of application remains fixed, with no consensus reached. The discussion remains unresolved as participants explore various examples and interpretations.

Contextual Notes

Participants note the importance of defining the reference frame and the conditions under which work is assessed, indicating potential limitations in understanding the problem without clear definitions.

ashtadmir
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If an object moves in such a way that the point of application remains fixed then what can be commented on the work done by the force? (only qualitative analysis required)
 
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Welcome to PF! Do you have an example in mind? This is typically a misunderstanding of the system definition and how the force is applied.
 
russ_watters said:
Do you have an example in mind? This is typically a misunderstanding of the system definition and how the force is applied.
No I don't. I was reading conceptual problems of a book when I got this TRUE/FALSE question "work done by a force on an object is zero, if the object moves in such a way that the point of application of the force remains fixed."
According to the answers this statement is False. I cannot think about any example or explanation for this.
 
Hmm. That doesn't sound right to me, but maybe it is just because I can't think of a good example. One common one is a person jumping up. The force is applied between the persons' feet and the ground and doesn't move. The Earth does no work, but it is a bit of a trick question because the person "deforms", releasing internal energy.

Maybe someone else can weigh-in...
 
ashtadmir said:
According to the answers this statement is False.
That doesn't sound right to me either. What book is this question from?
 
ashtadmir said:
the object moves in such a way that the point of application of the force remains fixed

Badly worded? Fixed relative to what?

If you push a book across a desk against friction the "point of application of the force" can remain fixed relative to the book but move relative to the table.
 
Doc Al said:
That doesn't sound right to me either. What book is this question from?
Doesn't sound right to be either. It is from a famous book in my country for preparation of admission to college. The name is "practice book of physics by DC Pandey". I don't think you would have heard of it.

CWatters said:
Badly worded? Fixed relative to what?

If you push a book across a desk against friction the "point of application of the force" can remain fixed relative to the book but move relative to the table.
I agree with you, the point of reference must be stated.

After thinking about the problem again I could think of one example that ought to satisfy the condition but proves the answer wrong: Consider a wheel fixed and hinged about its center, now if we bring it into contact with a conveyor belt then it would start rotating. Now since the point of application of force remains fixed and the center of mass does not move, we can say that the work done by that force is equal to zero.
Do you people agree with this explanation or something to add?
 
ashtadmir said:
Now since the point of application of force remains fixed and the center of mass does not move, we can say that the work done by that force is equal to zero.
Isn't the conveyor belt moving? And if the wheel starts rotating, something must have done work on it.
 
Doc Al said:
Isn't the conveyor belt moving? And if the wheel starts rotating, something must have done work on it.
The conveyor belt is moving but the point of application of force i.e the point of contact between the wheel and belt isn't.
I could be wrong about the work, but according to me since there is no net displacement of the wheel therefore work done is zero.
 
  • #10
I am a little surprised not to see Doc Al jump all over this.

A wheel on a conveyer belt is an excellent example of a point of application remaining fixed while non-zero work is done. Work is being done on the wheel because the material at the point of application is moving in the direction of the force applied to that material. Work is being extracted from the conveyer because the material at the point of application is moving in the opposite direction of the force applied to that material.

The position of the point of application is not directly relevant to the work done. It is the motion of the material to which the force is applied that matters.
 
  • #11
jbriggs444 said:
Work is being extracted from the conveyer because the material at the point of application is moving in the opposite direction of the force applied to that material.
That answers my question to satisfaction. Thank you.

PS. if a moderator sees this then you may close the thread.
 
  • #12
ashtadmir said:
The conveyor belt is moving but the point of application of force i.e the point of contact between the wheel and belt isn't.
Sure it is! The point of contact between wheel and belt does move (with respect to the room). I think what you're thinking is that, ignoring slipping between the surfaces, the relative motion of wheel and belt is zero. That's certainly true, and it just tells you that the "real" source of the work is whatever is moving the belt.

ashtadmir said:
I could be wrong about the work, but according to me since there is no net displacement of the wheel therefore work done is zero.
You are probably thinking of center of mass "work" (or pseudowork).
 
  • #13
jbriggs444 said:
I am a little surprised not to see Doc Al jump all over this.
I'm getting there... slowly. (Multi-tasking, and not very well.) :)
 
  • #14
Doc Al said:
Sure it is! The point of contact between wheel and belt does move (with respect to the room). I think what you're thinking is that, ignoring slipping between the surfaces, the relative motion of wheel and belt is zero. That's certainly true, and it just tells you that the "real" source of the work is whatever is moving the belt.You are probably thinking of center of mass "work" (or pseudowork).
I don't get it, how is the point of contact moving with respect to the room?
The real source of the work was not the problem, however I agree with you that the thing that is actually doing work is the motor that is moving the belt.
Yes earlier I was considering only the pseudowork (thanks for giving me the correct term) but i believe that jbriggs444 explained it clearly.
 
  • #15
ashtadmir said:
I don't get it, how is the point of contact moving with respect to the room?
Isn't the conveyor belt moving?

Contrast this with an example of a wheel rolling on the ground. In that case, the point of contact of the wheel is (momentarily) stationary. Not so with the conveyor belt.
 
  • #16
Doc Al said:
Isn't the conveyor belt moving?

Contrast this with an example of a wheel rolling on the ground. In that case, the point of contact of the wheel is (momentarily) stationary. Not so with the conveyor belt.

There is a distinction that can be drawn between the material at the point of contact and the point of contact itself. In the case of a car rolling along the road, the point of contact is moving but the material (both road and tread) is at least momentarily at rest. In the case of a wheel riding along a conveyer belt, the point of contact is stationary but the material (both belt and tread) is moving.

Which one of these situations applies is, of course, purely a matter of one's choice of reference frame. Regardless of that choice, it remains true that the motion of the material and of the point of contact are not identical.
 
  • #17
jbriggs444 said:
There is a distinction that can be drawn between the material at the point of contact and the point of contact itself.
I'm not quite clear what distinction you are making. When I speak of the "point of contact" of the wheel with the surface, I mean that physical portion of the wheel that is in momentary contact with the surface. What do you mean by "point of contact"?
 
  • #18
Doc Al said:
I'm not quite clear what distinction you are making. When I speak of the "point of contact" of the wheel with the surface, I mean that physical portion of the wheel that is in momentary contact with the surface. What do you mean by "point of contact"?
The point where contact is being made. The position of that point will be a function of time and might even be differentiable.
 
  • #19
jbriggs444 said:
The point where contact is being made. The position of that point will be a function of time and might even be differentiable.
Ah, now I see what you were saying. I was using "point of contact" as a shorthand way of referring to the physical portion of the wheel that is instantaneously in contact with the surface. And I see where my use of that term could lead to confusion. Thanks!
 

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