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It's not gravity but gravitational force.Actually the gravitational force acts as the centripetal force to keep the planets moving.But,that's not enough there is another force,that acts on a planet due to it's revolution around the sun, it's called centrifugal force.This force is equal and opposite to the gravitational pull of the sun (or,centripetal force).Thus,centrifugal force prevents the planets from colliding with the sun. Therefore, both centrifugal and gravitational force is required for the planets to move around the sun.
@gaos there is no such thing as centrifugal force - and it does not contribute to the rotation of the planets in their orbits.. If we assume for sake of simplicity that the orbits are circular (most in our solar system are very close to circular) we can look at the motion: A body will move in a straight line unless a force acts on it. In order to move in a circle a body needs a force acting TOWARDS the centre of the circle. (Think of the string when you whirl something round, or the arm of a centrifuge that they rotate astronauts in for tests...) This force creates the acceleration of the body to change the direction of its velocity. (In the case of the planets the magnitude of the velocity remains essentially constant) To move in a circular path the force acting towards the centre (the centripetal force) is mv^2/r where m is the mass of the body, v is its speed and r is the radius of the circular path 2 bodies are drawn to each other by the force of gravity (let's stick with the Newtonian explanation here....) which gets less as the distance between the objects increases (it get smaller with the SQUARE of the distance ) SO for a given planet with a given mass and a given speed there is one position relative to the sun where the force due to gravity is equal to the required centripetal force, and therefore the planet will orbit at THAT given radius from the sun. This is a gross over simplification of the real situation but explains the basic principle. The force due to gravity accelerates the planet towards the centre such that it moves in a curved (nearly circular) orbit...
Centripetal force due to gravitational force pull the earth toward the sun, centrifugal force, pull the the earth towarua tangent line on the orbit. If we sum both forces you get the approximately circle orbit of the earth as I've already said early ;)
@gaos No - that is not correct The centripetal force causes the planet to follow a curved path - it is accelerating all the time as a result of the centripetal force only. (IF there was such a thing as centrifugal force 'equal and opposite' then there would be no resultant force and the planet would travel in a straight line)
@gaos I see you have blocked me from discussing this with you in private message - but you must not insist on continuing to post your incorrect answers. I am sorry - I have to insist you are wrong. There is only the attractive force of gravity. If the planet was stationary then it WOULD fall into the sun, in th same way a rocket crashes back to earth if its engine fails on launch. It is only because the planet (or th satellite) has an tangential VELOCITY that it doesn't fall into th esun. THe force (and therefore the acceleration) is towards the centre, but the direction of travel is tangential. I cannot stress too strongly that you have misunderstood the dynamics of this. It is no concern of mine if you choose to ignore me, but if you truly have a desire to understand physics then you MUST review your understanding of this concept
I have looked at the link you sent me As with many answers that you find on this page I can only say 'you can't believe everything you find on the web' It is an incorrect explanation of the dynamics of orbit.
@gaos try this as a correct explanation: http://www.physicsclassroom.com/Class/circles/U6L4b.cfm