A frictionless spring with a 9-kg mass can be held stretched 0.4 meters beyond its natural length by a force of 30 newtons. If the spring begins at its equilibrium position, but a push gives it an initial velocity of 0.5 m/sec, find the position of the mass after t seconds.
a) x(t)= 0.4 cos (3t) +0.53sin(3t)
b)x(t)= 0.4 sin (3t) +0.5cos(3t)
c)x(t)= 0.4 cos (3t) -0.5sin(3t)
d)x(t)= 0.4 sin (3t) - 0.5cos (3t)
