A child is twirling a 0.0120-kg plastic ball on a string in a horizontal circle whose…

A child is twirling a 0.0120-kg plastic ball on a string in a
horizontal circle whose radius is 0.100 m. The ball travels once
around the circle in 0.500 s. (a) Determine the centripetal force
acting on the ball. (b) If the speed is doubled, does the
centripetal force double? If not, by what factor does the
centripetal force increase?
Question I need answered is, Why is gravity not considered when
solving this problem?