Final answer:
When thrust is less than drag, the correct answer is (b) Descent. For the airplane example with a drag of 40 kN and no other forces, kinetic energy will decrease, hence the answer is (b) decrease, 40 kN.
Step-by-step explanation:
When thrust is below drag, an aircraft experiences a decrease in velocity unless other forces act upon it to balance the forces. If thrust is not increased to match or exceed drag, the aircraft cannot maintain its speed or climb, thus the only option from the given answers is (b) Descent.
In the scenario of a 2000-kg airplane coming in for a landing without considering lift and with a drag force of 40 kN, the kinetic energy of the airplane will decrease due to the net force acting in the opposite direction of the motion. Therefore, the correct answer is (b) decrease, 40 kN.