Gravitational Force on Mass in Cavity of Planet

In summary, we use the formula F = Gm1m2/r^2 to find the gravitational force on a small mass m at the center of a spherical cavity in a large spherical planet of radius R made of a material of density d. By treating the cavity as filled in and adding a negative mass at the cavity, we can use the formula for the gravitational field inside a uniform spherical shell to find the force on m due to the complete sphere (S1) of radius R minus the complete sphere (S2) of radius R/2. This results in a force of 2GπdRm/3 on m at the center of the cavity.
  • #1
anshuman3105
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A large spherical planet of radius R made of a material of density d, has a spherical cavity of radius R/2, with center of cavity a distance R/2 from the centre of the planet. Find the gravitational Force on a small mass m at the centre of the cavity
 
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  • #2
anshuman3105 said:
A large spherical planet of radius R made of a material of density d, has a spherical cavity of radius R/2, with center of cavity a distance R/2 from the centre of the planet. Find the gravitational Force on a small mass m at the centre of the cavity
As per forum rules, you should quote any relevant standard equations of which you are aware and must show some attempt at a solution. If totally stuck, you should at least be able to provide some thoughts.
 
  • #3
using the formula F = Gm1m2/r^2, i am getting 16Gpidrm/3 but the solution is 2Gpidrm/2
 
  • #4
anshuman3105 said:
using the formula F = Gm1m2/r^2, i am getting 16Gpidrm/3 but the solution is 2Gpidrm/2
I get ##\frac 23 G\pi d r m## (I'm guessing the "/2" in what you posted is a typo).
Please post your working.
(There is a very quick method here.)
 
  • #5
Yes your answer is right...but how did you do it?
 
  • #6
I used Gm1m2/r^2
So F = (G(d*4/3pir^3)m)/(r/2)^2
 
  • #7
anshuman3105 said:
I used Gm1m2/r^2
That formula is essentially for point masses. It also works if one mass is a uniform spherical shell (or assembly of concentric uniform spherical shells) and the other (point) mass is outside all the shells.
The trick when dealing with cavities is to treat the cavity as filled in (i.e. no cavity) then add a 'negative mass' at the cavity. so in this case we have a complete sphere (S1) radius R minus a complete sphere (S2) radius R/2.
What do you know about the gravitational field inside a uniform spherical shell?
What is the force on m due to S1?
What is the force on m due to S2?
 
Last edited:
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Likes Saurabh
  • #8
Can you show it to me the solved part..?
 

Related to Gravitational Force on Mass in Cavity of Planet

1. What is gravitational force?

Gravitational force is a natural phenomenon that causes objects with mass to be attracted to one another. It is the force that keeps planets in orbit around the sun and objects on Earth from floating away into space.

2. How does gravitational force affect mass in a planet's cavity?

The gravitational force on an object inside a planet's cavity is determined by the mass of the planet, the mass of the object, and the distance between them. The closer the object is to the center of the planet, the stronger the gravitational force will be.

3. Does the shape of the cavity affect the gravitational force on an object?

Yes, the shape of the cavity can affect the gravitational force on an object. If the cavity is not spherical, the force of gravity will be stronger towards the center of mass of the cavity.

4. How is the strength of gravitational force on an object in a planet's cavity calculated?

The strength of gravitational force on an object in a planet's cavity is calculated using Newton's Law of Universal Gravitation, which states that the force of gravity is directly proportional to the product of the masses of the objects and inversely proportional to the square of the distance between them.

5. Can gravitational force be shielded in a planet's cavity?

No, gravitational force cannot be shielded in a planet's cavity. The force of gravity is a fundamental force of nature and cannot be blocked or shielded by any known material.

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