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## Main Question or Discussion Point

Dear Sir,

My name is Ashish R. Sai Im from India.I just appeared for 11th std.A few days ago I went through Gravitation.I completed my assignment.After completing assignments,I tried to find the gravitation between the moon and earth. During this i got a few doubts in this gravitation law.

1> My class sir told me that while dealing with planets we can consider planets as a point mass He told me that we can consider that that earth as an sphere because it got an imaginary boundary. My question is how can we consider the earth as an sphere when sphere got uniform density at each point But when we talk about earth itdont have same density at every point (I think so ).So how can we consider it as an sphere.

2>After this first I tried to find the force req for the motion of earth I thought it must be F=ma ,I found that mass is 5.9742 × 1024 and then i tried to find the 'a' of earth I found it in this way.

F=ma with the Newtonian law of gravity and the centripetal acceleration a = v2/r,

mv2r = −GMmr2.

This gives v = GM/r√, which is v = 29.5km/s or v = 2.95×104m/s, for the mass of the sun and r = 1.5×108km. The acceleration is then 5.8m/s2

Is it right if yes then the force req to move earth is f=ma which is equal to 3.48 × 1025

After this i found the force of gravitation between earth and moon which come as 1.9848 × 10^56 N ( By using ,)

How can be the gravitation between earth and moon be grater the force req to move earth so should earth move towards moon or moon should move towards earth.

3>Then I thought that not only moon is extracting force on earth other planets must be extracting some gravitational force on earth so somehow it must be balancing both forces.Am I right?

4> If yes then how can it be possible ? The number of planet is an constant so I think that the gravitational force extracted on earth and the gravitational force of moon.Both of them will not have an huge difference because even if we think that there is an planet out side of galaxy which have an huge size but in the formula of gravitation we know that there is distance so i mean to say when the size of plant is too large and it is located so far away from us and the distance comes in upon in this formula so it nullify the mass so the net force acting on earth and the net fore acting on moon will not have an huge difference .

5>If yes then why moon moves towards the huge gravitational force extracting plant so i think on moon earth must be the planet which extracting max gravitational force.So why moon is not moving towards earth.

My name is Ashish R. Sai Im from India.I just appeared for 11th std.A few days ago I went through Gravitation.I completed my assignment.After completing assignments,I tried to find the gravitation between the moon and earth. During this i got a few doubts in this gravitation law.

1> My class sir told me that while dealing with planets we can consider planets as a point mass He told me that we can consider that that earth as an sphere because it got an imaginary boundary. My question is how can we consider the earth as an sphere when sphere got uniform density at each point But when we talk about earth itdont have same density at every point (I think so ).So how can we consider it as an sphere.

2>After this first I tried to find the force req for the motion of earth I thought it must be F=ma ,I found that mass is 5.9742 × 1024 and then i tried to find the 'a' of earth I found it in this way.

F=ma with the Newtonian law of gravity and the centripetal acceleration a = v2/r,

mv2r = −GMmr2.

This gives v = GM/r√, which is v = 29.5km/s or v = 2.95×104m/s, for the mass of the sun and r = 1.5×108km. The acceleration is then 5.8m/s2

Is it right if yes then the force req to move earth is f=ma which is equal to 3.48 × 1025

After this i found the force of gravitation between earth and moon which come as 1.9848 × 10^56 N ( By using ,)

How can be the gravitation between earth and moon be grater the force req to move earth so should earth move towards moon or moon should move towards earth.

3>Then I thought that not only moon is extracting force on earth other planets must be extracting some gravitational force on earth so somehow it must be balancing both forces.Am I right?

4> If yes then how can it be possible ? The number of planet is an constant so I think that the gravitational force extracted on earth and the gravitational force of moon.Both of them will not have an huge difference because even if we think that there is an planet out side of galaxy which have an huge size but in the formula of gravitation we know that there is distance so i mean to say when the size of plant is too large and it is located so far away from us and the distance comes in upon in this formula so it nullify the mass so the net force acting on earth and the net fore acting on moon will not have an huge difference .

5>If yes then why moon moves towards the huge gravitational force extracting plant so i think on moon earth must be the planet which extracting max gravitational force.So why moon is not moving towards earth.