Why Does a Ruler Rise Up a Pencil When Swung at Higher Frequencies?

  • Context: Undergrad 
  • Thread starter Thread starter Lil_Aziz1
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Discussion Overview

The discussion revolves around the phenomenon observed when a ruler is swung around a pencil, specifically why the ruler rises up the pencil as the frequency of swinging increases. Participants explore the underlying physics concepts, including angular velocity, centripetal acceleration, and the forces at play.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant recalls that as frequency increases, angular velocity, angular acceleration, and centripetal acceleration also increase, but questions how this relates to the ruler rising up the pencil.
  • Another participant seeks clarification on whether the swinging occurs in a vertical or horizontal plane, indicating a need for a clearer understanding of the setup.
  • Some participants propose that the pencil traces out a cone during the swinging motion, suggesting that increased inertia may cause the ruler to move outward and upward on the pencil, counteracting the downward force of gravity.

Areas of Agreement / Disagreement

Participants express uncertainty about the mechanics involved and do not reach a consensus on the explanation for the ruler's behavior. Multiple viewpoints are presented regarding the forces and motion involved.

Contextual Notes

There is ambiguity regarding the exact motion of the ruler and pencil, as well as the specific forces acting on the ruler. The discussion does not resolve these uncertainties.

Who May Find This Useful

This discussion may be of interest to those studying dynamics, centripetal motion, or anyone curious about the physics of rotational systems.

Lil_Aziz1
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Hey everyone. So I was bored and I started swinging my ruler around my pencil (my ruler has a little hole where my pencil can be inserted into). Then I remembered we talked about why the ruler starts to go up the pencil if frequency increases in my physics class last year. The thing is, I can't remember why exactly this happens. I just remember talking about it. Now when frequency goes up, angular velocity goes up, then angular acceleration goes up, and then centripetal acceleration goes up. What's providing the centripetal acceleration? The normal force from the pencil (I think). However, this does not answer how it goes up the pencil because centripetal acceleration is horizontal and force of gravity is downward.

Can anyone help me out?

Thanks in advance!
 
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Erm. I'm having hard time picturing exactly what you are doing and what do you mean by going up. Do you mean swinging it in a vertical or horizontal plane?
 
Could it be that as you are twirling the ruler your pencil is tracing out a cone? As the speed increases, the inertia of the pencil creates more force force the ruler to move outward and upward on the pencil than gravity creates to move it downward.
 
skeptic2 said:
Could it be that as you are twirling the ruler your pencil is tracing out a cone? As the speed increases, the inertia of the pencil creates more force force the ruler to move outward and upward on the pencil than gravity creates to move it downward.

Exactly.
 

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