# Homework Help: Rotational problem

1. Jul 12, 2013

### Anthonyphy2013

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

A satellite follows an elliptical orbit. The only force on the satellite is the gravity atraction from the planet. The satellites speed at point A is 8000m/s, and it is 6000km away, Point B is 24000km east of the planet, and Point C is component vector 9000ikm+12000jkm away, if the you take the planet as the origin.

a. Is there any torque acting on the satellite?
b. satellite speed at Point B
c, satellite speed at Point C

2. Relevant equations

torque = force*R

3. The attempt at a solution
part a, I consider there is no torque since the only force is parallel to the pivot . Does it make sense.
part b ,since my professor does not mention gravitational force on this question , I use the f=mv^2/r to this question. i know there is no torque , and I know the radius is changing and I assume the gravitational force for at point a , b and c are the same and then I apply f=mva^2/ra = mvb^2/r2 . is that a good way to solve this problem ?

2. Jul 12, 2013

### pgardn

Have you guys covered conservation of angular momentum?

3. Jul 12, 2013

### Anthonyphy2013

yes , but how could angular momentum , L=mvr could apply to this question ?

4. Jul 12, 2013

### pgardn

Well the mass of the satellite does not change. And it has the same angular momentum at every place in its orbit with respect to the planet...

5. Jul 12, 2013

V*r = V'*r'

Familiar?