the other, soft and heavy. Why lead? It is because, in the child’s toy, the
constraint is different. Even a super-child cannot spin the flywheel of his toy up
to its burst velocity. The angular velocity ! is limited instead by the drive
mechanism (pull-string, friction drive). Then as we have seen, the best material
is that with the largest density. The second selection line on Figure 6.10 shows
the index M2 at the value 10 Mg/m3
. We seek materials in Search Area 2 to the
right of this line. Lead is good. Cast iron is less good, but cheaper. Gold,
platinum, and uranium (not shown on the chart) are better, but may be thought
unsuitable for other reasons.