# Ly need help on gravitation only one part

1. Jan 15, 2010

### squintyeyes

urgently need help on gravitation only one part

Comets travel around the sun in elliptical orbits with large eccentricities. If a comet has speed 7.0 x 10^4 m/s when at a distance of 2.5 x 10^11 m from the center of the sun, what is its speed when at a distance of 9.0 * 10^10 m?
___________ m/s

Attempt
(7.0 x 10^4)(2.5 x 10^11)= v(9.0 * 10^10)
v = 194444.444

However, this answer was marked wrong.

2. Jan 16, 2010

### ehild

Re: urgently need help on gravitation only one part

First: in what units is your v given?

The angular momentum is constant along the orbit, but its magnitude is r*v* sin(angle between the radius and the direction of the velocity). This angle is unknown. So use the conservation of momentum instead.

ehild

3. Jan 16, 2010

### squintyeyes

Re: urgently need help on gravitation only one part

v is in m/s. And what do you mean by saying that i should use coservation of momentum. Isn't that what i did but it was marked wrong?

4. Jan 16, 2010

### ehild

Re: urgently need help on gravitation only one part

Sorry, I meant conservation of energy...

ehild

5. Jan 16, 2010

### squintyeyes

Re: urgently need help on gravitation only one part

okay but if i use conservation of energy i still encounter a problem.

KE + PE = KE +PE
1/2mv^2 + (-GMm/r) = 1/2mV^2 + (-GMm/R)
1/2v^2 + (-GM/r) = 1/2V^2 + (-GM/R)

I know
v = 2 x 10^4 m/s
r= 3 x 10^11 m
R=4 x 10^10 m

but i still need to know what M, the mass of the comet is equal to.

6. Jan 16, 2010

### JaWiB

Re: urgently need help on gravitation only one part

Isn't M the mass of the sun?