As we know gravitational force is a central force so due to gravitational effect they must collide with each other but why they revolve?
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$\begingroup$ Who says they "must". Newton became famous for explaining how the central force of gravity causes Keplerian orbits. $\endgroup$– John AlexiouCommented Feb 5, 2017 at 17:52
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The incidence of falling apple inspired Newton to unify the of gravity on earth and the gravity in space. Both an apple and the moon "falling" to Earth by the same kind of force. The moon has large enough "horizontal" velocity to prevent it from "crashing" the earth but changes its direction all the time.
answered Feb 5, 2017 at 17:53
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3$\begingroup$ It's generally frowned upon to provide answers to questions which are this obviously duplicates of previous questions. $\endgroup$ Commented Feb 5, 2017 at 17:54
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$\begingroup$ Does it have any velocity component towards earth? $\endgroup$ Commented Feb 5, 2017 at 17:59
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$\begingroup$ In reality, the moon's orbit is elliptic (to certain extend), so the moon sometimes moving up and sometimes moving down unless at the asides. For perfect circular orbit, the gravity is merely providing the centrepetal force, that'll be no radial component of velocity. $\endgroup$ Commented Feb 5, 2017 at 18:08