Question

why a jet aeroplane releases a lot of hot gases before it takes off ??

Answer 1

A jet engine actually has two main sections, a compressor for the intake air and a turbine that is driven by the burning gases that also turns the compressor. So the initial turning of the engine is to get all of this working, up to speed and up to operating temperature. The hot gases leaving the engine have a high velocity, pushing forward the engine and the aircraft it is attached to. Since the aircraft is at a standstill before take-off, a lot of energy is needed to get it up to take-off speed, that energy is supplied by those high velocity hot gases leaving the engine.