What is it called when you do not have enough fissionable material to maintain or start a nuclear chain reaction?

The diagram shows the positions of the sun, moon and earth during spring tides, when the high tides are at their highest and low tides at their lowest. what is it about these positions that causes these high and low tides?

Arollercoaster car at the top of a hill has potential energy kinetic energy chemical energy light energy

How many moles of air are there in a human lung with a volume of 2.4 l at stp? explain your answer