Physics (acceleration)?

Question

A lunar landing craft descends vertically towards the surface of the moon with a constant speed of 2.0m/s. The craft and crew have a total mass of 15,000kg. Assume that acceleration due to gravity on the moon is 1.6m/s^2.
(a) During the first part of the decent, the upward thrust of the rocket engines is 24,000N. Show that this results in the craft moving at a constant speed.
(b) The upward thrust of the engine is increased to 25,500N for the loast 18 seconds of the descent.
i. Calculate the deceleration of the craft during this time
ii. What is the speed of the craft just before it lands?
iii. How far is the craft above the surface of the moon when the engine thrust is increased to 25,500N?

Answer 1

Answer 1 had the idea but used the wrong numbers.
(a) acceleration a = f - mg = 24000 - 15000*1.6 = 0
(b.i) a = (25500 - 24000)/m = 0.1 m/s^2 (using up=+ convention)
(b.ii) vf = vi + at = -2.0 + 0.1 * 18 = -0.2 m/s
(b.iii) h = -(vi * t + 0.5 at^2) = 36 - 16.2 = 19.8 m

Answer 2

a. For constant speed, there must be no net force.
....15000kg * 1.6 m/s = 24000 N

b. a = F/m
...a = (25000 - 25500)/15000 = -0.03 m/s

...vf = vo + at = 2 - 0.03*18 = 1.46 m/s

...x = vt + 1/2at^2 = 1.46*18 + 0.03*18^2 = 36m