# How to calculate the comet's speed?

1. Dec 9, 2006

### jkh4

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

Comet moves around the sun in a very ellipical orbit around the sun. As it approach, in 1986, Comet Halley was 8.79x10^7km from the sun and moving with a speed of 54.6km/s. What will be the comet's speed be when it crosses Neptune's orbit in 2006?

length of comet from the sun = 8.79x10^7km = 8.79x10^10 m
v of comet = 54.6km/s = 54.6x10^3m/s
T = 20 years
length of neptune from the sun = 4.50x10^12m

2. Relevant equations

i use the equation T^2 = (4*pie*r^3)/GM and K2 + U2 = K1 + U1

3. The attempt at a solution
i first use the Kelper's rule to find M of the comet, but i don't really know if that's the right step to start with.

Thank you.

2. Dec 9, 2006

### OlderDan

Make the substitutions in your conservation of energy equation and see if you really need to know the mass of the comet.

3. Dec 9, 2006

### jkh4

one question, when we calculate with the K2 + U2 = K1 + U1, the M we are using, is that the mass of the sun or neptune?

4. Dec 9, 2006

### OlderDan

The mass of Neptune does not matter. You are just using the radius of the Neptune orbit as a distance from the sun. The potential energy terms depend on the mass of the sun and the mass of the comet.

5. Dec 9, 2006

### jkh4

i got the answer, thank you so much!

btw, so does that mean if for similiar questions, the M is always M of sun?

Last edited: Dec 9, 2006
6. Dec 9, 2006

### OlderDan

If the orbit is an orbit around the sun, then the mass of the sun is involved. There are many problems involving orbits around other force centers, so M will not always be the sun's mass.