Derivation of Kepler's laws- differential equation question

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SUMMARY

The discussion centers on the differential equation u" + u = km/L^2, which is relevant in deriving Kepler's laws. The solution is expressed as u(θ) = km/L^2 + C cos(θ - θ₀), where C is a constant and θ is the independent variable. The general solution combines the homogeneous solution u₀ = C cos(θ - θ₀) with the particular solution uₚ = km/L². This method of combining solutions is standard in linear differential equations with non-zero additional terms.

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ddoctor
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Hi group,

Could someone 'remind me' why the equation u" + u = km/L^2 has the solution of the form u(theta) = km/L^2 + C cos(theta - theta(o)).
Any references would be appreciated.
Thanks

Dave
 
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a linear differential equation with non zero additional term has a soulution which is a sum of a general solution of this equation with zero additional term and a particular solution of this equation. To be more clear (I assume that theta is independent variable) the equation

u^{''}+u=0
has solution
u_0=C\times cos(\theta-\theta_0)

on the other hand, your equation has a particular solution

u_p=km/L^2=const
so the general solution is

u=u_0+u_p
 

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