Been reading about kinetic energy weapons, and it got me thinking,

say a 200 tonne passenger jet crashed into London at Mach 8, what would the resulting explosion/shock wave be like. Would it have the potential to flatten London, or would it be just a big ball of flames?

Also, during the planes descent, would it speed up due to gravity or slow down due to a loss of thrust and friction?

11 hours ago, Curious layman said:

 

Also, during the planes descent, would it speed up due to gravity or slow down due to a loss of thrust and friction?

Mach 8 is going to be above the terminal velocity, so assuming it's no longer is powered, it will be slowing down regardless of angle of descent.

10 hours ago, Curious layman said:

Been reading about kinetic energy weapons, and it got me thinking,

say a 200 tonne passenger jet crashed into London at Mach 8, what would the resulting explosion/shock wave be like. Would it have the potential to flatten London, or would it be just a big ball of flames?

Also, during the planes descent, would it speed up due to gravity or slow down due to a loss of thrust and friction?

The kinetic energy would be 1/2 mv^2 

Mach 8 is 2744 m/s

If that's at elevation, yes there will be more energy owing to the potential energy, but it's 2 x 10^6 joules/meter, which is small compared to the KE and, as you say, some will be lost to air resistance 

7.5 x 10^11 Joules, or 0.18 kilotons of TNT equivalent, or ~1/83 of the Hiroshima bomb yield