# Gravitational force between a particle and sphere

• UrbanXrisis
In summary: So the force isF = \frac{GM_{1}M_{2}b} {r^3}In summary, the only correct option for the gravitational force equation is c, and the reasoning behind it is that the net force on a particle inside a hollow shell is zero, as there is no mass contained inside the smaller sphere. This is also analogous to the gravitational effect of a mass inside a solid sphere.

#### UrbanXrisis

the question is http://home.earthlink.net/~urban-xrisis/phy002.jpg [Broken]

I thought that...
$$F=G\frac{m_1m}{R_1^2}-G\frac{m_2m}{R_2^2}$$
a.
$$F=G\frac{m_1m}{a^2}-G\frac{m_2m}{(R_2-a)^2}$$
b.
$$F=G\frac{m_1m}{b^2}-G\frac{m_2m}{(R_2-b)^2}$$
c.
$$F=G\frac{m_1m}{c^2}+G\frac{m_2m}{c^2}$$

is this correct?

Last edited by a moderator:
The only correct one is c.

For a and b, note that the mass is inside the shell, consider the acceleration from the mass ofthe shell closest to it in comparison to the mass farthest of it. Is there a net effect?

Last edited:
when the mass is in the middle of the shell, it has a net force of zero, but I don't know how to calculate the force when it is not in the center of the shell

It's very similar to the reasoning behind the gravitational effects of a mass inside a solid sphere. Tell me what the gravitational effect for a mass inside a sphere is, and think of the reasoning why it is so. Take a guess as to how this applies for a shell.

for gravitational effect inside a shere...

$$F=G\frac{mMr}{R^3}$$

where r<R

I'm not sure how this relates to a shell

Ugh, I had this whole thing typed out and something went wrong and lost it.

Anyway, for a particle (at radius $r$) inside a solid sphere of radius $R$, the net force of gravitaiton on the particle results only from the mass contained inside the sphere with radius $r$.

what about the gravity inside a shell? it's not the same as a sphere is it?

Same principle applies, except there's no mass in the smaller sphere

$$F=G\frac{m_1mr}{R_1^3}-G\frac{m_2mr}{R_2^3}$$
a.
$$F=G\frac{m_1ma}{R_1^3}-G\frac{m_2ma}{R_2^3}$$
b.
$$F=G\frac{m_1m}{(R_1+b)^2}-G\frac{m_2mb}{R_2^3}$$

Reconsider what I said about a particle inside a hollow shell

And for B, check your radius, R_1 + b doesn't make sense.

are you saying it should be:
a.
$$F=G\frac{m_1ma}{R_1^3}$$
b.
$$F=G\frac{m_1m}{b^2}$$

Yeah, do you understand why?

but the formula: $$F=G\frac{mMr}{R^3}$$

it doesn't include the mass of the inner sphere...so it can't equal zero

m- is the point mass
M- mass of sphere/shell
R- radius of sphere/shell

also, what about the force pulling it towards the shell? as in since it's in the shell, would the shell exert a force towards the shell's surface? hence the negative force? (that's why I subtracted)

The force pulling it towards the shell cancels in the horizontal direction due to symmetry, in the vertical direction, the forces cancel since there's a certain amount of mass on top of the particle at a small radius away, but there's also a lot more mass at a farther radius. These two end up cancelling also, so everything cancels.

That formula is for a uniform sphere. The shell isn't a uniform sphere.

It is also analogous to

$$F = G\frac{mM_{contained}}{R^2}$$

$M_{contained}$ being the sphere that is below the point mass? which in my case would = 0?

Yes, with respect to the hollow shell, it contains no mass inside the sphere with radius b, therefore exerts no net force.

Mass is proportional to radius. So M2/radius 2=Mcountained/b. So the force that the big mass exerts is

if M=Particle mass
M2=Big sphere mass

That is why you get this equation.

Last edited:
The big sphere density is:

$$\frac{3M_{2}} {4 r^3 {\pi}}$$

and since the volume of the sphere with radius b is

$$\frac{4 b^3 {\pi}} {3}$$

So the mass is

$$M_{countained} = \frac{M_{2}b^3} {r^3}$$

since

$$a = \frac{M_{countained}G} {b^2}$$

$$a = \frac{G M_{2} b^3} {r^3 b^2}$$

Simplifying

$$a= \frac{G M_{2} b} {r^3}$$

Last edited:

## 1. What is the equation for calculating gravitational force between a particle and a sphere?

The equation for calculating gravitational force between a particle and a sphere is F = G(m1m2)/r^2, where F is the gravitational force, G is the gravitational constant, m1 and m2 are the masses of the particle and sphere respectively, and r is the distance between the two objects.

## 2. How does the distance between the particle and sphere affect the gravitational force?

The gravitational force between a particle and a sphere is inversely proportional to the square of the distance between the two objects. This means that as the distance increases, the force decreases and vice versa.

## 3. What is the role of the mass of the objects in determining the gravitational force?

The masses of the objects play a crucial role in determining the magnitude of the gravitational force between them. The larger the masses, the stronger the force will be.

## 4. Can the gravitational force between a particle and a sphere be negative?

No, the gravitational force between a particle and a sphere can never be negative. It is always a positive value, as the force is always attractive between two objects.

## 5. How does the strength of the gravitational force change if one of the masses is increased?

If one of the masses is increased, the gravitational force will also increase. This is because the force is directly proportional to the masses of the objects. So, as one of the masses increases, the force will increase proportionally.

• Introductory Physics Homework Help
Replies
23
Views
237
• Introductory Physics Homework Help
Replies
1
Views
153
• Introductory Physics Homework Help
Replies
8
Views
830
• Introductory Physics Homework Help
Replies
20
Views
936
• Introductory Physics Homework Help
Replies
2
Views
507
• Introductory Physics Homework Help
Replies
7
Views
1K
• Introductory Physics Homework Help
Replies
6
Views
1K
• Introductory Physics Homework Help
Replies
4
Views
975
• Introductory Physics Homework Help
Replies
4
Views
952
• Introductory Physics Homework Help
Replies
6
Views
372