Zhang Tie's flight speed was 3,487 m/s, which was close to 10 times the speed of sound.
The speed of spear that Zhang Tie could throw at rest was 1,689 m/s, which was close to 5 times the speed of sound.
When in flight, the moment the spear left Zhang Tie's hand, although its speed being relative to Zhang Tie was 1,689 m/s, its real speed was 5,176 m/s, ignoring the gravity acceleration from over 1,000 m high.
When the spear hit the building, its kinetic energy was 750,147,328 Joules. The moment the spear left his hand, Zhang Tie had got its kinetic energy through his powerful mental arithmetic by abacus.
According to the standard before the Catastrophe, the kinetic energy of over 750 million Joules was almost equal to the explosive quantitive of 180 kg's TNT.
Of course, in this age, there was no explosive except for alchemist's bomb; additionally, the strike of kinetic weapon was more concentrative and more destructive to a single target.
