Friction in a hemispherical bowl

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Discussion Overview

The discussion revolves around the dynamics of a point mass sliding with friction in a hemispherical bowl, starting from rest at the top. Participants explore the equations of motion, the effects of friction, and methods for approximating the speed of the mass as it descends.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents the equations of motion, incorporating friction (f=μN) and gravitational forces, and expresses uncertainty about finding a simple solution.
  • Another participant suggests that an exact solution would require solving a differential equation, but proposes an approximation method for small μ that involves calculating speed without friction and adjusting for energy loss due to friction.
  • A third participant agrees with the approximation approach and notes the presence of coupled second-order nonlinear equations, indicating a strategy to use frictionless speed to estimate energy lost.
  • Another participant calculates an approximate speed at the bottom of the bowl based on the frictionless case and compares it to a different incline scenario, showing how friction affects the results.
  • One participant shares simulation results that support the approximations made, noting energy values for different coefficients of friction and identifying a critical value of μ where the mass stops at the center of the bowl.
  • A final participant expresses appreciation for the findings and indicates plans to share the results on their website, acknowledging the contributors without providing explicit links.

Areas of Agreement / Disagreement

Participants generally agree on the complexity of the problem and the utility of approximations for small μ, but there is no consensus on a definitive solution or method, as different approaches and results are discussed.

Contextual Notes

The discussion involves assumptions about the smallness of μ and the applicability of certain approximations. The relationship between friction and energy loss is also explored, but specific mathematical steps and dependencies remain unresolved.

f todd baker
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I have been trying to find the speed of a point mass sliding with friction (f=μN) in a hemispherical bowl. Start at the top at rest. So far I have -μN+mgcosθ=ma and N=mgsinθ+mv2/R where θ is the angle below horizontal from the center of the sphere and R is the radius of the sphere. I know it likely does not have a simple solution but would be happy with an approximate solution for small μ.
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For an exact solution you'll have to solve a differential equation.

For an approximation for small μ, you can calculate the speed without friction, then determine friction and energy loss based on that speed, then calculate a new speed based on the initial value and the energy loss. That gives an easier integral instead of a differential equation.
 
Right you are, and as I see it there are two coupled second-order, nonlinear equations. Good idea to use v(θ,μ)≈v((θ,0) to calculate energy lost. I'll try it.
 
I think the suggestion by mfb was a good one. Using the frictionless v(θ)=√(2gRsinθ) I find v≈√(2gR(1-3μ)) at the bottom. For comparison, a path on a 450 incline with the same drop has v=√(2gR(1-μ)).
 
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Out of curiosity, I simulated the setup. With μ=0.001 and m=R=g=1 I got E=0.99701, in agreement with your result. μ=0.01 leads to E=0.9704. Even with μ=0.1, the approximation is not bad: E=0.732.

The mass stops at the center for μ>0.60. Tested with 100, 200 and 500 steps: The critical value is somewhere between 0.603 and 0.605. There is no obvious mathematical constant in that range.
 
This is beautiful. I will post your results on my web site. (I have had my wrist slapped here before by mentioning it explicitly, so will not give you the exact link!) I will link back here so that you will be properly acknowledged. Thanks.
 

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