# Speed of comet

1. Sep 27, 2007

### jamagner

A comet is in an elliptical orbit around the Sun. Its closest approach to the Sun is a distance of 4.5 1010 m (inside the orbit of Mercury), at which point its speed is 8.9 104 m/s. Its farthest distance from the Sun is far beyond the orbit of Pluto. What is its speed when it is 6 1012 m from the Sun? (This is the approximate distance of Pluto from the Sun.)

i have no clue how to find the speed given only the distances and the other speed

2. Sep 27, 2007

### Staff: Mentor

Hint: What's conserved?

3. Sep 27, 2007

### jamagner

see thats what i dont know...my guess would be kinetic energy is conserved but i dont really know...i suck at physics i never know whats going on

4. Sep 27, 2007

### Staff: Mentor

Mechanical energy (kinetic energy + gravitational potential energy) is conserved.

5. Sep 27, 2007

### jamagner

so how do i use this in an equation can i set it up as a proportion

6. Sep 27, 2007

### Staff: Mentor

Initial total energy = final total energy. You'll need to know how to calculate gravitational PE.

7. Feb 19, 2009

### sriceb01

What if you dont know the mass of the comet, because dont you need this to use the gravitational PE?

8. Feb 19, 2009

### Staff: Mentor

You don't need the actual mass. Just call it "m" and see what happens.