Confusion About Oscillating Mass

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Discussion Overview

The discussion revolves around the dynamics of a mass oscillating on a crankshaft when an external force is applied and subsequently removed. Participants explore the implications of frictionless motion, the role of torque, energy conservation, and the effects of mass distribution in the system. The conversation touches on theoretical aspects of oscillation, periodic motion, and the conditions necessary for sustained oscillation.

Discussion Character

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

Main Points Raised

  • Some participants suggest that the system will continue to oscillate at the same frequency even after the applied torque is removed, assuming no energy loss.
  • Others question the meaning of "rate" in the context of oscillation and whether gravity should be considered in the analysis.
  • A participant proposes that the motion will be periodic, while another emphasizes the importance of the center of mass in the system's dynamics.
  • There is a debate about whether momentum can be conserved if the center of mass moves, with some asserting that it must be conserved regardless of the system's configuration.
  • One participant calculates the torque required for oscillation and expresses confusion about the necessity of torque when oscillation is assumed to be maintained without energy loss.
  • Another participant points out contradictions in assuming massless components can store energy, leading to questions about the feasibility of such a system.
  • Some participants discuss the need for a heavy flywheel to maintain motion beyond a certain point, while others argue that any mass should theoretically allow for constant frequency oscillation.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for sustained oscillation, the role of mass in the system, and the implications of energy conservation. There is no consensus on the correct interpretation of the dynamics involved, and multiple competing views remain throughout the discussion.

Contextual Notes

Participants highlight limitations in their assumptions, particularly regarding the mass of components and the implications for energy storage and conservation. The discussion remains unresolved regarding the precise mechanics of the oscillating system.

  • #31
No need for numbers . Algebra variables are what we will use . We can keep the actual maths quite simple .

Getting a bit late now here in UK so I'm signing off pro tem .

Nos dda .
 
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  • #32
Nidum said:
No need for numbers . Algebra variables are what we will use . We can keep the actual maths quite simple .
Seems simple enough. Finite mass on the piston, rigidly mounted crank-case/engine block. Frictionless bearings and frictionless piston rings. Watch what happens as the mass of the crankshaft and connecting rod decrease toward zero. There is always an increase in crankshaft angular velocity near top dead center and bottom dead center. As the mass of the crankshaft and connecting rod decrease toward zero, the peak angular velocity diverges toward infinity.

There is no convergence toward a physically reasonable limiting behavior. Something will break first.
 
  • #33
jbriggs444 said:
Seems simple enough. Finite mass on the piston, rigidly mounted crank-case/engine block. Frictionless bearings and frictionless piston rings. Watch what happens as the mass of the crankshaft and connecting rod decrease toward zero. There is always an increase in crankshaft angular velocity near top dead center and bottom dead center. As the mass of the crankshaft and connecting rod decrease toward zero, the peak angular velocity diverges toward infinity.

There is no convergence toward a physically reasonable limiting behavior. Something will break first.
All right. My train of thought before making the thread was this:
I know that when the mass is momentarily still, the acceleration is at its maximum. I can find that acceleration, and then find what the angular acceleration would be at that point. This would allow me to find the maximum torque. It did give me an equation (τ=ω^2l^2m) which passes dimensional analysis and seems reasonable, but I never considered the mass of the rod. Does this calculation make any sense to do, or should I go back to the drawing board completely?
 
  • #34
Also what is exactly meant by:
jbriggs444 said:
There is no convergence toward a physically reasonable limiting behavior. Something will break first.
What is meant by something breaking? Is this meant in a physical sense?
 
  • #35
person123 said:
Also what is exactly meant by: What is meant by something breaking? Is this meant in a physical sense?
How are you going to manage infinite acceleration with a material with a finite strength to mass ratio?
 
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  • #36
jbriggs444 said:
How are you going to manage infinite acceleration with a material with a finite strength to mass ratio?
I see what you meant.
 

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