Why does an airplane have a "flight ceiling," a maximum altitude above which it can't obtain enough lift to balance the downward force of gravity? Consider the discussion about what happens to air at high altitude. 2. If you put your hand out the window of a moving car, so that your palm is pointing directly forward, the force on your hand is directly backward. Explain why the two halves of the airstream, passing over and under your hand, don't produce an overall up or down force on your hand. 3. If instead of holding your hand palm forward (see question 2), you tip your palm slightly downward, the force on your hand will be both backward and upward. How is the airstream exerting an upward force on your hand? 4. When a plane enters a steep dive, the air rushes toward it from below. If the pilot pulls up suddenly from such a dive, the wings may abruptly stall, even though the plane is oriented horizontally. Explain why the wings stall. 5. What is the benefit of a turbofan engine as compared to a turbojet engine?