A model rocket is made up of a shuttle of mass 20 kg, and a fuel tank which
has a mass of 4 kg when it is empty. When the rocket attains an upward
velocity of 10 ms, all of its fuel is used up. At this point, the empty tank is
fired downwards at a velocity of 5 ms. This causes the rocket to increase its
upward velocity to 13 ms Calculate the change in momentum of:
a) the rocket without empty fuel tank (60 kg m s'upward)
b) the empty fuel tank. (60 kg m s' downward)
Explain why the velocity of a rocket increases when a fuel tank is fired
downwards​