# Homework Help: Comets Velocity

1. Oct 21, 2007

### Jtappan

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

The orbit of Halley's Comet around the Sun is a long thin ellipse. At its aphelion (point farthest from the Sun), the comet is 5.7 * 10^12 m from the Sun and moves with a speed of 11.0 km/s. What is the comet's speed at its perihelion (closest approach to the Sun) where its distance from the Sun is 8.4 * 10^10 m?
_____km/s

2. Relevant equations

PEi+KEi=PEf+KEf

3. The attempt at a solution

do the m and g values cancel out when doing this because they are not given? Im totally lost...

2. Oct 21, 2007

### mgb_phys

Yes, KE = 1/2 m v^2 and PE = M m /r where M is the mass of the sun.

3. Oct 21, 2007

### mdk31

Kepler's 2nd Law is equivalent to the conservation of momentum.

r[a]mv[a]sin(x) = r[p]mv[p]sin(x)

As you can see, the mass of the central body is irrelevant and the masses of the comet cancel out.

4. Oct 21, 2007

### Jtappan

what is the meaning of sin(x) in both of these? it gives no angles

5. Oct 22, 2007

### mdk31

The momentum (mv) is perpendicular to the comet's path. Therefore, the angle between the r position vector and the momentum vector is 90. The sine of 90 is 1.

Last edited: Oct 22, 2007
6. Oct 22, 2007

### Jtappan

ok I tried that and got 746.4285 and that is still not the right answer...I dont know what I am doing wrong....

7. Oct 22, 2007

### mdk31

Do you get the same answer when you try conservation of energy?

8. Oct 22, 2007

### Mindscrape

Oh, you made another topic. mdk is right, $r_a m v_a = r_p m v_p$ so you must have misinterpreted something he said or done the numbers wrong.