b)

The particle stops when its speed is zero.

> speed := simplify( sqrt( V[1]^2 + V[2]^2 + V[3]^2 ) );

speed := sqrt(10+4*t^2-12*t)

> solve( speed=0, t );

3/2+1/2*I, 3/2-1/2*I

>

Since there are no REAL-valued times t when the speed is zero, the particle never stops.

>