**1. The problem statement, all variables and given/known data**

In the Virial theorem The scalar virial G is defined by the equation:

$$G=\vec{p}\cdot \vec{r}$$

Where ##\vec{p}## is the momentum vector and ##\vec{r}## the location vector.

When i take the mean of the derivative ##\bar{\dot{G}}## over a whole period T it equals 0. why?

**2. Relevant equations**

$$\vec{p}\cdot \vec{r}=(mv)\cdot \cos \theta \cdot r$$

**3. The attempt at a solution**

I understand this scalar product is zeroed during one period, but why?

http://ift.tt/1mdVJVs