B How does the Paper Spiral work?

  • B
  • Thread starter Thread starter phantomvommand
  • Start date Start date
  • Tags Tags
    Paper Spiral Work
AI Thread Summary
The discussion centers on how the paper spiral operates, particularly the role of hot air rising from a candle and its effect on generating rotational torque. Participants debate the mechanics of air movement and the angle of the paper blades, comparing the spiral's function to helicopter blades. They clarify that the blades are not vertical but diagonal, allowing for both vertical and horizontal forces to contribute to rotation. Misunderstandings about the nature of the normal contact force are addressed, emphasizing that it is always perpendicular to the surface. Overall, the mechanics of the paper spiral involve both the angle of the blades and the interaction with rising air.
phantomvommand
Messages
287
Reaction score
39
Hot air rises due to the candle below.
But how does that create a horizontal force, which is needed to produce the rotational torque?
 
Physics news on Phys.org
Are you asking about something similar to this?
1659120153648.png


Maybe you can post a picture of the paper thing you're asking about.
 
  • Like
Likes phantomvommand
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
 
  • Like
Likes phantomvommand
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
 
  • Like
Likes phantomvommand, hutchphd and malawi_glenn
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.
 
  • Like
Likes phantomvommand and malawi_glenn
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/
 
  • Like
Likes phantomvommand

Similar threads

Replies
37
Views
2K
Replies
7
Views
212
Replies
11
Views
6K
Replies
9
Views
2K
Replies
1
Views
1K
Back
Top