Physics. The speed of a child riding a merry-go-round at a carnival is given by the equation $v=\sqrt{12 r},$ where $v$ is the speed in feet per second and $r$ is the distance in feet from the center of the merry-go-round to the rider. If a child is moving at $15 \mathrm{ft} / \mathrm{s}$ how far is the child from the center of the merry-go-round?