Earth's gravitational force is acting on its Moon in such a way that it forces the Moon to rotate round its orbit by centripetal force and carries it while rotating round the Sun by gravitational force. I don't understand why in this condition Moon doesn't come to the Earth while Earth is carrying it through gravitational force to rotate round the Sun?

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    $\begingroup$ Related: physics.stackexchange.com/q/9049 $\endgroup$
    – ghoppe
    Feb 19, 2013 at 22:33
  • $\begingroup$ I'm a bit confused by the question. As I understand it, @ghoppe 's related link answers the question. Is there another question in your question that isn't addressed there? $\endgroup$ Feb 19, 2013 at 23:01
  • $\begingroup$ @joshphysics I don't understand that a body is moving somewhere and another body at a distance of 384000km away from that body is going with it because of the attraction of the moving body. i don't understand if the earth has such much power that it is carrying moon everywhere it is going then why can the earth not attract moon in such a way that it attracts to a falling apple. I mean why does moon (in in condition) come to the earth and collide with it when earth has such much power that the moon goes with it where ever earth goes $\endgroup$
    – kashif
    Feb 19, 2013 at 23:42
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    $\begingroup$ @kashif The linked related question answers that question. The moon is always "falling" towards the earth like your apple, it's just "falling" at the precise distance where it misses the ground. $\endgroup$
    – ghoppe
    Feb 20, 2013 at 0:00

2 Answers 2


I think I might understand another facet of your question besides what is addressed in the comments. Let me demonstrate a result in classical mechanics which I think might alleviate your concern.

The result is that

Given a system of particles, the center of mass of the system moves is though it were a point mass acted on by the net external force on the system.

So if you think of the Earth-Moon system as being acted on by a net external force which is simply the gravitational attraction to the Sun (to good approximation), then what's happening is that this entire system is orbiting (essentially freely falling) around the sun. The details of what's happening in the Earth-Moon system itself are described by the first link in the original comments, but for purposes of what's happening to the entire system consisting of the Earth+Moon when it orbits the Sun, the details of the internal interactions don't really matter.

Here is a proof of the statement above:

Consider a system of particles with masses $m_i$ and positions $\mathbf x_i$ as viewed in an inertial frame. Newton's second law tells us that the net force $\mathbf F_i$ on each particle is equal to its mass times its acceleration; $$ \mathbf F_i = m_i \mathbf a_i, \qquad \mathbf a_i = \ddot{\mathbf x}_i $$ Let $\mathbf f_{ij}$ denote the force of particle $j$ on particle $i$, and let us break up the force $\mathbf F_i$ on each particle into the sum of the force $\mathbf F^e_i$ due to interactions external to the system and the net force $\sum_j \mathbf f_{ij}$ due to interactions with all other particles in the system; $$ \mathbf F_i = \mathbf F_i^e + \sum_j \mathbf f_{ij} $$ Combining these two facts, we find that $$ \sum_i m_i\mathbf a_i = \sum_i \mathbf F_i^e + \sum_{ij} \mathbf f_{ij} $$ The last term vanishes by Newton's third law $\mathbf f_{ij} = -\mathbf f_{ji}$. The term on the left of the equality is just $M\ddot {\mathbf R}$ where $M$ is the total mass and $\mathbf R$ is the position of the center of mass of the system. Combining these facts gives $$ M\ddot{\mathbf R} = \sum_i \mathbf F_i^e $$


Gravity is applied to objects even when they don't touch the earth. For example, think about a stone. Toss it in the air, and it comes back down, even though it was in the air, not touching the earth.

The earth's gravity is strong, strong enough to pull on the moon and not let it get away. The moon is about 238,900 miles (384,400 km) from the earth. The moon's gravity, even so far away, is still strong enough to pull on the earth, and cause the tides to rise and fall in the oceans.

Like you said, centripetal force is trying to pull the moon down onto the earth. But centrifugal force is trying to send the moon flying in a straight line and off into space. The two forces are equal, so the moon is caught, orbiting the earth.

The sun is much larger; its gravitational pull has all the planets caught and orbiting it.


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