How does the Paper Spiral work?

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    Paper Spiral Work
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Discussion Overview

The discussion revolves around the mechanics of how a paper spiral operates, particularly focusing on the forces involved in its rotation and the role of air movement. Participants explore the relationship between vertical airflow and the generation of horizontal torque necessary for rotation, examining similarities to helicopter blades and other demonstrations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that hot air rising from a candle creates a force necessary for the spiral's rotation, questioning how this results in horizontal torque.
  • Others draw parallels between the paper spiral and helicopter blades, noting that while helicopter blades push air diagonally to create horizontal torque, the paper spiral's design may lack this horizontal component due to the vertical nature of the airflow.
  • A participant argues that the angle of the blades in the paper spiral contributes to the generation of horizontal force, prompting a consideration of impulse direction during collisions with the blades.
  • Another participant asserts that the blades are not vertical but diagonal, indicating that the forces involved include both vertical and horizontal components.
  • One participant challenges the assertion that the normal contact force is purely vertical, emphasizing that it is always perpendicular to the surface, thus suggesting a more complex interaction than initially proposed.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the mechanics of the paper spiral and the forces at play. No consensus is reached on the nature of the forces involved or the correctness of specific claims about the normal contact force.

Contextual Notes

Participants reference specific illustrations and external resources to support their arguments, indicating a reliance on visual aids and examples that may not be universally understood. The discussion also highlights potential misunderstandings regarding the mechanics of forces and motion.

phantomvommand
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Hot air rises due to the candle below.
But how does that create a horizontal force, which is needed to produce the rotational torque?
 
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Are you asking about something similar to this?
1659120153648.png


Maybe you can post a picture of the paper thing you're asking about.
 
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anorlunda said:
Are you asking about something similar to this?
View attachment 305014

Maybe you can post a picture of the paper thing you're asking about.
Yes, although the usual demonstration is less elaborate.

Some demonstrations like these https://www.ingridscience.ca/node/706 remind me of helicopter blades. But the difference I see here is that while the helicopter blades push the air in a diagonal direction such that there is a horizontal torque, in the paper spiral, when the vertically ascending air particles are stopped by an infinitesimally small paper area, the normal contact force is vertical (thus lacking the crucial horizontal component)
 
phantomvommand said:
But how does that create a horizontal force
Because of the angle of the blades.
Forget about the air, let a particle collide with the blade instead. Seen from the side:
What is the direction of the impulse on the blade?
1659120670335.png
 
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phantomvommand said:
when the vertically ascending air particles are stopped by an infinitesimally small paper area, the normal contact force is vertical (thus lacking the crucial horizontal component)
No, the paper blades in the illustrations you linked are not vertical, they are diagonal. Part of the force is vertical, part horizontal.

1659120892406.png
 
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Are you talking about something like this, @phantomvommand? The principle is the same as the helicopter blades - the flat spiral falls into a spiral ramp, so the paper is everywhere slanted downwards in the same direction (clockwise or anticlockwise). The diagram in @malawi_glenn's post #4 then applies, and you get rotation.
 
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phantomvommand said:
... in the paper spiral, when the vertically ascending air particles are stopped by an infinitesimally small paper area, the normal contact force is vertical (thus lacking the crucial horizontal component)
That statement is not correct.
The normal contact force is always perpendicular to the surface.

Please, see #1 here:
https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/
 
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