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`sqrt(20gR)``sqrt(21gR)``sqrt(17gR)`None of these

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The ball must strike the sphere normally (i.e. along radius) so that is can rebound back along the same path <br> `sintheta = (R//2)/(R) = (1)/(2)` <br> `theta = 30^(@)` <br> The velocity of the ball at the instant of hitting the sphere is inclined at `theta` to the horizontal <br> `:. tantheta = (v_(y))/(v_(x))` <br> `(1)/(sqrt(3)) = (sqrt(2g((7R)/(2))))/(u)` <br> `:. u = sqrt(21gR)` <br> <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/RES_P07_17_APCT_P7_XI_E01_003_S01.png" width="80%">