Usain Bolt, a Jamaican sprinter, holds the Olympic and world records for the 100-m and 200-m dash, which he achieved during the 2008 Summer Olympics in Beijing, China. He finished the 200-m dash in 19.30 seconds.
What was his speed?

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.