4 A particle moves in a straight line. Its displacement \(t\) seconds after leaving the fixed point \(O\) is \(x\) metres, where \(x = \frac { 1 } { 2 } t ^ { 2 } + \frac { 1 } { 30 } t ^ { 3 }\). Find
- the speed of the particle when \(t = 10\),
- the value of \(t\) for which the acceleration of the particle is twice its initial acceleration.