In a circular orbit, the only force required to stay in the circle is a centripetal force towards the center. To understand why this works, the definition of force as stated in Newton’s second law ($F=m\cdot a$) comes in to be very useful. Force causes an acceleration towards the direction of the force, changing the velocity direction and leading to a circular motion. The theory of circular motion has many parallels to the theory of linear motion, and learning it results in a more complete understanding of mechanics.

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