A car of mass $m$ accelerates from rest along a straight horizontal track. Its engine delivers a constant power $P$. When it has traveled a distance $D$, its momentum is $p$. A second, heavier car of mass $8m$ also accelerates from rest along the same track, with its engine delivering the exact same constant power $P$. What is the momentum of the heavier car after it has traveled the same distance $D$? Ignore all resistive forces.(Answer in term of $p$)