141. How does a Frisbee fly?
As you begin to move a Frisbee forward, the air in front of the Frisbee splits to flow either over the Frisbee or under it. Because of the Frisbee's shape and the angle at which it's held, the air that flows over the Frisbee has a longer distance to travel and arrives late at the back of the Frisbee. The air flowing under the Frisbee reaches the back first and initially flows upward, around the rear surface of the Frisbee. But once the Frisbee is moving fairly rapidly, this funny upward-flowing tail of air blows away from the back of the Frisbee. As it leaves, it draws the air flowing over the Frisbee with it and speeds that air up. As a result, the air over the Frisbee travels faster than the air under the Frisbee. But the airs above and below the Frisbee have the same amounts of total energy per gram. Since the faster moving air above the Frisbee has more kinetic energy than the slower moving air below the Frisbee, the air above the Frisbee must have less of some other form of energy than the air below the Frisbee. In fact, the air above the Frisbee has less pressure potential energy than the air below it—the air pressure above the Frisbee is less than that below the Frisbee. And since the pressure pushing on the bottom surface of the Frisbee is greater than the pressure pushing on the top surface of the Frisbee, there is a net upward pressure force on the Frisbee. This upward pressure force balances the downward weight of the Frisbee and keeps the Frisbee from falling.