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Gravity is related to mass, and gravity is an attracting force on every body. Then what happens when bodies only attract each other..without repulsion they may strike the sun. I think there must be a repulsive force.

Another question arises, if only attraction is there, then why does the moon rotate elliptically around the earth?

And Mars comes closer to earth after 3 years?

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Gravity does not repel, but there is another "force" at work here. The angular momentum of the original protoplanetary disk is conserved. In other words, just because the cloud condensed into planets doesn't mean it is going to stop spinning.

When we orbit artificial objects, we speed them up until they reach a velocity that sustains orbit--so again there is another force at work, just not gravity. Gravity keeps objects in orbit (rather than letting them fly off into space). Imagine if you spin a yo-yo around in the air. The tension of the string is like gravity, keeping the yo-yo in orbit, but if you let go of the string then the other "force" (its momentum) will cause it to fly off.

(Technically, momentum is not a force. It is the product of the mass and velocity of the system--a conserved quantity, but the object required a force to bring it up to that velocity. In the case of a planet, it is the astronomical forces that acted on the protoplanetary disk. In the case of the yo-yo, it was your applied force to spin it.)

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It's simply that

we are moving fast.

It's that simple. The Earth is moving VERY fast around the sun, so it is not "dragged in".

If for some bizarre reason the Earth slowed down ... it would move in closer to the Sun. If it stopped moving - it would fall directly in to the sun. It's that simple.

Note that this is exactly how satellites work. (I mean the satellites we launch to use for TV communications and the like.) If they need the satellite to move down closer, they simply slow it down a bit. To go up higher, you just speed it up a bit.

So it's that simple.

"I think there must be a repulsive force."

There's no repulsive force ... we are quite simply moving very fast!

Note that, quite simply, say you want a rocket to "get away from" the gravitational power of the Earth ... what do you do? Simply make it go very fast! It's that simple.

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