Assuming comets are evenly distributed we can find first the volume of a sphere with the formula frac{{4}{3}}πr³ using 50000 as the radius, and then dividing this volume by the amount of comets. This gives a the space each comet has on average.
We apply the cube root as proposed in the question.
The kinectic energy is given <em>K=frac{{1}{2}}mv² where m is the mass and v is the velocity. Apropriate unit are given in the question.</em>
