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Homework Statement
The problem reads: "If the Earth were suddenly to stop in its orbit, how long would it take for it to collide with the Sun?[Regard the Sun as a fixed point mass. You make use of the formula for the period of the Earth's orbit.]" (The answer is 65 days. Obviously, I'm not interested in the numbers, I want to know where I am going wrong.)
Homework Equations
Newton's Law:
[tex]m_{earth} \frac{dv}{dt} = -G m_{earth} m_{sun} \frac{1}{r^{2}}[/tex]
which implies
[tex]\frac{dv}{dt} = -G m_{sun} \frac{1}{r^{2}}[/tex]
A change of variable from t to r:
[tex]\frac{dv}{dt} = \frac{dv}{dr}\frac{dr}{dt} = v \frac{dv}{dr}[/tex]
The Attempt at a Solution
So far, I am getting (plugging in the change of variable)
[tex]v \frac{dv}{dr} = -G m_{sun} \frac{1}{r^{2}}[/tex]
Separate the variables.
[tex]v dv = -G m_{sun} \frac{dr}{r^{2}}[/tex]
The next step is to integrate. v is integrated over 0 (initial time) to t (t being the time of impact). My question right now is what is r integrated over? If I use R (the "radius" of the circular path) to 0, I end up with a 1/0 term (obviously wrong).
[tex](1/2) v^{2} - (1/2) v_{0} ^{2} = G m_{sun} [\frac{1}{0} - \frac{1}{R}][/tex]
Obviously, I am stuck there, but to give you an idea of where I am going, I will set the initial velocity (Vo) to 0 (the Earth stopped). Then I will change v to dr/dt. Separate Variables and integrate again. Then solve for time, t.