# Position and Velocity Vector

1. Jan 1, 2009

### 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).

2. Jan 1, 2009

### arildno

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