A 12,000-kg airplane is in level flight at a speed of 870 km/h. What is the upward-directed liftforce exerted by the air on the airplane?

To solve this problem, we need to understand that in level flight, the lift force (F) exerted by the air on the airplane is equal to the weight (W) of the airplane. The weight of an object is its mass (m) times the acceleration due to gravity (g).

Here are the steps to solve this problem:

1. Convert the speed of the airplane from km/h to m/s because the standard unit of speed in physics is m/s.

   870 km/h = 870,000 m/3600 s = 241.67 m/s

2. Calculate the weight of the airplane. The standard value of acceleration due to gravity is 9.8 m/s².

   W = m * g = 12,000 kg * 9.8 m/s² = 117,600 N

3. In level flight, the lift force is equal to the weight of the airplane.

   F = W = 117,600 N

So, the upward-directed lift force exerted by the air on the airplane is 117,600 Newtons.