Final answer:
The maximum airspeed with the landing gear extended, known as VLE, varies by aircraft and is specified by the manufacturer. If a drag force acts rearward on a plane during landing, the plane's kinetic energy would decrease due to this opposing force.
Step-by-step explanation:
The maximum airspeed with the landing gear extended and locked is not universally defined as it can vary based on the type of aircraft. In general, this speed is determined by the aircraft manufacturer and is included in the aircraft's operating manual as part of the Aircraft Flight Manual (AFM) or Pilot's Operating Handbook (POH). This speed is referred to as the maximum landing gear extended speed (VLE). The correct answer to this question would depend on the specific details provided by the aircraft's operational guidelines or by the context of the question if this is a theoretical situation. Unfortunately, without a specific aircraft model or additional context, only a general statement can be made.
For the scenario where kinetic energy changes due to a net force on a landing airplane, by considering the net force to be in the direction opposite the motion (drag force), kinetic energy would decrease. If the airplane is coming in for a landing with a drag force of 40 kN, then the net force is in the direction opposite to the motion, which means that the kinetic energy of the airplane would decrease by this force.
The answer options provided (a. increase, 20 kN; b. decrease, 40 kN; c. increase, 45 kN; d. decrease, 45 kN) seem to be incomplete in explaining the actual physics of the scenario. However, the presence of a drag force typically indicates the loss of kinetic energy, hence 'decrease' would be the most logical option.