Is the Velocity Vector Perpendicular to the Position Vector?

[Tasell]

A point M is located by the vector r(t), which depends on time, but the length of r(t) is constant. Show that the velocity v(t) of M is perpendicular to r(t).

 

[arildno]

Well, what can you say about the quantity:
$$\vec{r}\cdot\vec{r}$$