Solving a Particle's Spiral Orbit under Central Force

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SUMMARY

The discussion centers on deriving the force equation for a particle in a spiral orbit defined by r=a*theta under a central force. The correct force expression is f(r) = (-L^2/mr^3)*[1 + (a^2)/(r^2)]. A participant struggles with an extraneous factor of 2 in their calculations, specifically when applying the differential equation d^2u/dtheta^2 + u = (-1/ml^2u^2)*f(u^-1). The issue arises during the substitution of the second derivative of u with respect to theta.

PREREQUISITES
  • Understanding of central force motion
  • Familiarity with polar coordinates and spiral orbits
  • Knowledge of differential equations
  • Proficiency in classical mechanics concepts, particularly angular momentum
NEXT STEPS
  • Review the derivation of forces in polar coordinates
  • Study the application of the differential equation d^2u/dtheta^2 + u = (-1/ml^2u^2)*f(u^-1)
  • Examine the relationship between angular momentum and central forces
  • Explore common pitfalls in solving differential equations in mechanics
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Students and educators in physics, particularly those focusing on classical mechanics and central force problems, as well as anyone interested in advanced mathematical techniques in physics.

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Homework Statement


A particle moves under the action of a central force in a spiral orbit given by r=a*theta. Show that the force is f(r) = (-L^2/mr^3)*[1 + (a^2)/(r^2)]


Homework Equations





The Attempt at a Solution


I know how to do the problem, but I keep ending up with a factor of 2 for the a^2 term. Any pointers on how to fix this? Thanks for the help!
 
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Sorry for that... I used the equation d^2u/dtheta^2 + u = (-1/ml^2u^2)*f(u^-1). For the second derivative of u with respect to theta i got 2*a*theta^-3, which i re-wrote as 2*a^2*u^3. I plugged that result into the diff. equation, but was unable to 'get rid of' the factor of 2 for the a^2 term. Any pointers?
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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