A particle is moving in a circle of radius $r$ under the action of a force $F = \alpha r ^ { 2 }$ which is directed towards centre of the circle. Total mechanical energy (kinetic energy + potential energy) of the particle is (take potential energy $= 0$ for $r = 0$ ):\\
(1) $\frac { 5 } { 6 } \alpha r ^ { 3 }$\\
(2) $\alpha r ^ { 3 }$\\
(3) $\frac { 1 } { 2 } \alpha r ^ { 3 }$\\
(4) $\frac { 4 } { 3 } \alpha r ^ { 3 }$