Velocity of Comet at a given distance

[Hamish Taylor]

Homework Statement

Halley’s Comet travels in a highly eccentric (non-circular) orbit. At its closest approach, it is about 9×10^7 km away from the sun and travels with a speed of 54.6 km/s. What is the comet’s speed when it crosses the orbit of Neptune, about 5 billion kilometres from the sun?

Homework Equations

Conservation of Energy, Ei = Ef

The Attempt at a Solution

I have made numerous attempts utilised conservation of energy because I don't see why we would need to bring angular momentum into it (given that we're not told that 5 billion km is the apopasis and therefore can't reduce it to mvr), why does my conservation of energy equation give me an answer of 8.3 km/s when the answer should be 0.8 km/s? Any help would be much appreciated. Whenever I insert the provided answer into the conversation of energy formula, it just doesn't work out.

 

[jbriggs444]

I have made numerous attempts utilised conservation of energy because I don't see why we would need to bring angular momentum into it (given that we're not told that 5 billion km is the apopasis and therefore can't reduce it to mvr), why does my conservation of energy equation give me an answer of 8.3 km/s when the answer should be 0.8 km/s? Any help would be much appreciated. Whenever I insert the provided answer into the conversation of energy formula, it just doesn't work out.

We might be better able to find errors in your work if you were to show your work. Set angular momentum to one side and show us one attempt at a conservation of energy approach.

 

[ehild]

Homework Statement

Halley’s Comet travels in a highly eccentric (non-circular) orbit. At its closest approach, it is about 9×10^7 km away from the sun and travels with a speed of 54.6 km/s. What is the comet’s speed when it crosses the orbit of Neptune, about 5 billion kilometres from the sun?

Homework Equations

Conservation of Energy, Ei = Ef

The Attempt at a Solution

I have made numerous attempts utilised conservation of energy because I don't see why we would need to bring angular momentum into it (given that we're not told that 5 billion km is the apopasis and therefore can't reduce it to mvr), why does my conservation of energy equation give me an answer of 8.3 km/s when the answer should be 0.8 km/s? Any help would be much appreciated. Whenever I insert the provided answer into the conversation of energy formula, it just doesn't work out.

You are right, conservation of energy should work. I got nearly the same result as you.