well u x v = 0

you know |u x v | = |u||v|sin(b)

b being the angle between the 2 vectors.

So that means that b has to be 0 or 180 degrees.

you know that u * v = |u||v|cos(c)

where c is the angle between the 2 vectors.

So if the dot product is equal to zero, then it means that the angle between the 2 vectors is 90 or 270.

can you take it from there?

then the second question you have

v(t) is the first derivative of r(t), and a(t) is the second derivative of r(t)