# Homework Help: Force on Particle in Dust Cloud

1. Mar 14, 2008

### cameo_demon

[SOLVED] Force on Particle in Dust Cloud

The following problem is from Thorton & Marion's Classical Dynamics, Ch. 5 Problem 5-13 (p. 205 in the 5th edition of the text)

1. The problem statement, all variables and given/known data
A planet of density $$\rho_{1}$$ (spherical core, radius $$R_{1}$$) with a thick spherical cloud of dust (density $$\rho_{2}$$, radius $$R_{2}$$) is discovered. What is the force on a particle of mass $$m$$ placed within the dust cloud?

2. Relevant equations
$$V_{sphere}=\frac{4}{3}\pi \ r^{3}$$
$$F = \frac{-GmM}{r^{2}}$$
$$\rho = \frac{m}{v}$$

3. The attempt at a solution
So my intuition for this one is to solve for big M and add the mass of the cloud with the mass of the planet.
$$M_{1} = \frac{4}{3}\pi\rho_{1} \ {R_{1}}^{3}$$
for the mass of the planet, and:
$$M_{2} = \frac{4}{3} \pi\rho_{2} {R_{2}}^{3}$$

substituting $$M$$ with $$M_{1} + M_{2}$$ and a bit of factoring, I get:
$$F = \frac{4}{3} \frac{Gm \pi ({R_{1}}^{3}\rho_{1} + {R_{2}}^{3}\rho_{2})}{r^{2}}$$
Yet somehow this doesn't feel right...

The text provides answers for the even numbers only, so I don't know how to verify this. I feel like there's something else I should be doing and it might involve calculus...

Any suggestions? Thanks in advance for any help.

2. Mar 14, 2008

### Dick

Well, the dust cloud isn't in the planet and you should only be using the mass of the dust cloud inside of the radius of the particle. So maybe it would be better to use (r-R1) instead of R2?

3. Mar 14, 2008

### cameo_demon

Ah, alright!

So the mass of the dust cloud (as far as we are concerned, which is the radius of the particle) is now:
$$M_{2} = \frac{4}{3} \pi\rho_{2} (r-R_{1})^{3}$$

$$F = \frac{4}{3} \frac{Gm \pi ({R_{1}}^{3}\rho_{1} + (r - R_{1})^{3}\rho_{2})}{r^{2}}$$

and I'm assuming there should be a few things that end up cancelling when all is said and done.

Is there anything else, or is thing solved?

4. Mar 14, 2008

### Dick

You know what? That's not right either. You only want the mass of the dust cloud between you and the planet. Make that (r^3-R1^3)*rho2. Sorry. Don't close the thread and call it solved until you are happy. Obviously, I make mistakes. Don't agree with me too fast.

Last edited: Mar 14, 2008
5. Mar 14, 2008

### rl.bhat

If the partical is at a distance r such that R1<r<R2, the froce on the partical due to the planet and dust cloud can be calculated by finding the mass of the sphere of radius r.
F = 4/3*G*m*pi*[R1*3 + (r-R1)^3]/r^2

6. Mar 14, 2008

### Dick

Great. You made the same mistake I did.

7. Mar 15, 2008

### rl.bhat

Yes Dick, I realised it later on.

8. Mar 15, 2008

### cameo_demon

Yes I think that makes more sense. Thanks for your help!