# Homework Help: Speed of a comet around the sun.

1. Feb 18, 2010

### Overseer

1. The problem statement, all variables and given/known data

A comet is in an elliptical orbit around the Sun. Its closest approach to the Sun is a distance of 4.5e10 m (inside the orbit of Mercury), at which point its speed is 9.6e4 m/s. Its farthest distance from the Sun is far beyond the orbit of Pluto. What is its speed when it is 6e12 m from the Sun? (This is the approximate distance of Pluto from the Sun.)

2. Relevant equations

3. The attempt at a solution

I don't know where to start with this. All i'm told is its speed at the "bottom" of its orbit, and then i'm told to calculate its speed at the "top" of it orbit. I am not given the mass of the comet so I can't find it's Potential or Kinetic Energy which is what this section is over. To my knowledge there is no direct relationship between distance in an orbit and a bodies speed. Perhaps I'm overlooking something brutally obvious.

2. Feb 18, 2010

### dacruick

you need to use potential energy. because you know its speed at a certain distance, mass becomes a common factor and you no longer need it. So you have the speed at a certain distance, and you need to find its speed at a further distance.

analogy:

a ball of mass m is thrown into the air and has an instantaneous speed right before it hits the ground of 35 metres per second. What is its speed when it is a distance of 15 metres from the ground.

You can use energy to find this answer, and it is exactly the same concept. good luck