The period of oscillation of a bob in an accelerating frame

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Discussion Overview

The discussion revolves around the period of oscillation of a pendulum bob in an accelerating frame of reference, such as inside a moving car. Participants explore the effects of fictitious forces and how they influence the pendulum's behavior, particularly focusing on the time period of oscillation and the effective acceleration due to gravity in this non-inertial frame.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that in an accelerating frame, the pendulum bob experiences a fictitious force that causes it to hang at an angle to the vertical, raising the question of how to determine the time period of oscillation.
  • Another participant questions what the effective value of "g" would be in the accelerating frame, particularly how an object would behave if dropped in that frame.
  • Some participants propose that the y acceleration remains as "g" while considering the reaction force from the car's acceleration, suggesting that the time period could still be expressed as 2π√(l/g).
  • There is a challenge to the assertion that the time period remains 2π√(l/g), with a participant arguing that the fictitious force alters the effective "g".
  • One participant describes drawing diagrams to analyze the forces acting on the bob, indicating that the horizontal components of the gravitational force and fictitious force cancel out, leading to a focus on tension and the resultant forces.
  • Another participant encourages a step-by-step analysis using Newton's second law to compare the forces acting on the pendulum in both the stationary and accelerating scenarios, suggesting that the resulting equations will lead to simple harmonic motion (SHM) about the equilibrium position.

Areas of Agreement / Disagreement

Participants express differing views on how the fictitious force affects the effective acceleration due to gravity and the resulting time period of oscillation. There is no consensus on the correct formulation for the time period in the accelerating frame, and the discussion remains unresolved.

Contextual Notes

Participants reference the need for diagrams and detailed force analysis, indicating that assumptions about the forces and their interactions may not be fully resolved. The discussion also highlights the complexity of applying Newton's laws in non-inertial frames.

Soffie
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If a suspended pendulum bob is accelerated (in a car, for example), if you're in the accelerating frame of reference, you will observe the fictitious force which appears to act on the bob (as you're in the accelerating frame, the bob is not 'moving' so to speak, so to establish equilibrium you introduce the fictitious force.
The bob is thus at an angle to the vertical, due to the fictious force in the accelerating frame, OR due to the acceleration in the inertial frame. If the bob performs small oscillations about the line the angle makes to the vertical, how would you go about finding the time period? Presumably it'd not just sqrt(l/g)
 
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Soffie said:
Presumably it'd not just sqrt(l/g)
In that accelerating frame, what would the effective value of "g" be? (If you drop something, how would it accelerate as measured in that frame?)
 
Last edited:
Sum the forces on the bob. The y acceleration would still be g. And the other force would be a reaction from the car accelerating. I think it still would be 2*pi*sqrt(l/g).
 
osilmag said:
Sum the forces on the bob. The y acceleration would still be g. And the other force would be a reaction from the car accelerating.
Only two "real" forces act on the bob: gravity and the tension in the cord. But viewed from the accelerating frame there is an additional "fictitious" force that must be added. The effect of that added force can be viewed as a change in the effective "g".

osilmag said:
I think it still would be 2*pi*sqrt(l/g).
That is not correct.
 
Doc Al said:
Only two "real" forces act on the bob: gravity and the tension in the cord. But viewed from the accelerating frame there is an additional "fictitious" force that must be added. The effect of that added force can be viewed as a change in the effective "g".That is not correct.
Ok so I've drawn some diagrams which may help solve the problem- I've oriented the axes so the tension is pointing up- what the bob looks like in its effective equilibrium position, and resolved the fictitious force and the weight in terms of the angle phi, which was the angle made due to the acceleration: (sorry I think you have to view the images full size down below)
206628-4e0ca3aac3a018d12cddbf1a1e339fa6.jpg

Now here's the diagram for the displacement:
206629-ecf3704eee6272619b8401bae6a43f20.jpg

am I right in thinking the horizontal components of mg and Ffict still cancel out, so we're just left with tension and the downward force due to F fict and mg? Then you can just solve using SHM equations.
 

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I'm not quite sure I follow what you're doing. Try this. Imagine the pendulum hanging from the ceiling of a train car. First, assume no acceleration. What forces act? Apply Newton's 2nd law. I'm sure you understand how that situation can lead to SHM about the equilibrium position.

Next, have the car accelerate. Viewed from within the accelerating car, what forces act? Apply Newton's 2nd law. (You'll need to add the fictitious force to apply Newton.) How can you rearrange the resulting equation to make it look similar to the previous one? (And thus convince yourself that it will also lead to SHM about the equilibrium position. The same equations have the same solutions!)
 

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