Comet Orbit Around the Sun: Calculating Speed

[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

 

[Doc Al]

Hint: What's conserved?

 

[jamagner]

see that's what i don't know...my guess would be kinetic energy is conserved but i don't really know...i suck at physics i never know what's going on

 

[Doc Al]

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

 

[jamagner]

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

 

[Doc Al]

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

 

[sriceb01]

What if you don't know the mass of the comet, because don't you need this to use the gravitational PE?

 

[Doc Al]

What if you don't know the mass of the comet, because don't you need this to use the gravitational PE?

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