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Homework Help: Central Force problem

  1. May 21, 2007 #1
    1. The problem statement, all variables and given/known data

    Central force F=-ar/(r^3) & Central potential energy,U=-(a/r)
    (not U_eff)
    Find the nature of orbits if (i)a>0 and (ii)a<0

    2. Relevant equations
    3. The attempt at a solution

    If we remember the attractive central force E=E(r) diagram,i.e.the one showing the graph of U_eff,we only need to know E_total=K+U.
    Where only PE is given.
    We see,

    U= -integration[F.dr]=integration[dW]=-integration[dK]=-K

    Then K=a/r and U=-(a/r)

    So,the E=K+U=0

    Then in positive and negative both caes we get a parabolic orbit.

    Please check if i am correct.
  2. jcsd
  3. May 21, 2007 #2
    Consider a>0.
    Then, -(m(v^2)/r)=-(a/r^2)
    Hence total energy E=K+U= -(1/2)(a/r)

    =>elliptic orbit.

    Consider a<0.

    Then,F=+b/r where b=-a>0
    and U_p=b/r

    K is itrinsically positive.So,total energy positive.

    From e=sqrt[1+{(2L^2*E)/(m*b^2)}]
    hence hyperbolic trajectory.
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