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musicmar
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Homework Statement
At its perihelion in February 1986, Comet Halley was 8.79x10^7 km form the Sun and was moving at 54.6 km/s. What was its speed when it crossed Neptune's orbit in 2006?
Homework Equations
K0+U0=K+U
The Attempt at a Solution
I know I need to do this using conservation of energy. I have v-initial and r-initial, and presumably I can look up r-final. The mass of the comet will cancel, and big M will be the mass of the sun(also can look up). I am just confused about the r's, though. I was wondering why it is necessary to know the dates or that it was at its perihelion. How do I find the distance it was from the sun when it crossed Neptune's orbit?