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FranzDiCoccio
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Homework Statement
The period of a comet is 75.8 years. The perihelion distance is 0.596 AU (1 AU = 1.5 ⋅ 1011 m).
The velocity at perihelion is vp = 5.45 ⋅104 m/s.
a) Find the length of the major semi-axis of the elliptical orbit.
b) Find the aphelion distance and the velocity at aphelion for the comet.
Homework Equations
T2 = k a3 (Kepler's third law)
m v r = const. (conservation of angular momentum at aphelion and perihelion)
[tex]1/2 m v^2 - \frac{G m M}{r} = const[/tex] (conservation of energy)
The Attempt at a Solution
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I used the 3rd Kepler's law to find the semi-major axis a. For the constant I used
[tex]k=\frac{4 \pi^2}{G M}[/tex]
where M = 1.99 ⋅1030 kg as provided by the book.
I find
a = 17.9 AU = 2.68⋅1012 m
ra = 35.2 AU = 5.28 ⋅1012 m
Then, from [tex]m v_a r_a = m v_p r_p[/tex] I find va=923 m/s.
So far, I'm in complete agreement with the solution provided by the book.
Now, I think that the same result could be derived from the conservation of energy.
[tex]v_a = \sqrt{v_p^2+2GM \left(\frac{1}{r_a}-\frac{1}{r_p}\right)}[/tex]
However, this formula gives a completely different result: 7.15 ⋅103 m/s.
Much larger than the previous one.
For the life of me, I cannot see the mistake. Here we are talking about a small comet orbiting the Sun, so the latter should be basically at rest. Center of mass should not be relevant.
The other thing I can think of is the constant in Kepler's law. But that should be ok in this case (again, the comet has a very small mass)
Can anyone help? Could it perhaps be a matter of roundoff? Difference between very small numbers?
EDIT: corrected typo in initial formula for energy
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