# Homework Help: Retarded Force in Space and interstellar dust

1. Jul 9, 2009

### whiztle

1. The problem statement, all variables and given/known data
A uniform, spherical planet of mass M and radius R moves SLOWLY with an essentially uniform speed v through a cloud of interstellar dust particles, whose density is ρ. The dust particles are attracted towards the planet, and some of them would eventually fall onto its surface.

Find the resulting retarding force on the planet due to the dust cloud.?
Since the planet moves slowly, initial speed and final speed can be assumed to be the same

2. Relevant equations
Angular momentum => Li = Lf
Momentum => Pi = Pf
Energy including
Potential energy = GMm/R2
Kinetic Energy = 1/2 (mv2)

3. The attempt at a solution

The clue is to use conservation of momentum, angular momentum, and energy.
Solution should be in terms of speed v, radius R, mass M, and density ρ.

2. Jul 10, 2009

### diazona

Hmm... well, the clue you give suggests that maybe this isn't what is intended, but are you familiar with this equation?
$$F = \frac{\mathrm{d}p}{\mathrm{d}t}$$
That's my first thought...