This is another one that came out of teaching the introductory physics class.
Whenever we cover angular momentum, we always end by discussing gyroscopes. A gyroscope is a wheel with a rigid, low-friction axis. It has most of its weight concentrated on the outside, so it is rotationally "heavy". Angular momentum is a vector (i.e. the direction of rotation matters). The purpose of a gyroscope is to carry a large quantity of angular momentum at safe rotation speeds and to be physically aligned with its angular momentum vector. This sets up a great classroom demonstration of the three-dimensional nature of torque. (Torque is defined as the rate of change of angular momentum; it is also a vector.) Any force used to try to reorient the gyroscope produces a torque that is perpendicular to both the force applied and the axis of the gyroscope. Students have been told that this is true repeatedly and even done homework calculations, but when the whole gyroscope kicks sideways and twists out of their grip it becomes much more real.
The wonky dynamics of a gyroscope when large torques are applied make them fun demonstrations. The other side of constrained angular momentum is that small bumps and twists have almost no effect on the gyroscope's orientation. Even if a gyroscope moves side to side violently, it will usually continue pointing in the same direction. Among other things, this is incredibly useful for navigation. Most planes and ships today have a gyroscope somewhere on board in an isolated environment. The gyroscope is not mechanically tied to the vessel. Instead as the vessel pitches and yaws the apparent motion of the gyroscope indicates the attitude deviance, allowing for rapid and precise corrective action. Attitude (meaning rotational orientation) control is critical to navigation, especially in air travel. If a plane is not flying level, a course correction intended to steer left or right may include a downward component as well, a potentially disastrous error.
We are used to thinking of up as "the direction opposite the pull of gravity" but in stormy seas or high winds this metric loses its meaning. When the world around has lost all of its familiar reference points, a gyrostabilizer acts as an internal "external reference frame". Navigation in any direction remains possible because there is something that is inside the vessel, but unaffected by its turbulent environment, that knows which way is up.
A gyroscope by itself won't keep a plane from crashing. The pilot needs to note that his attitude is out of synch with the reference and take corrective action. Actually, the pilot rarely has to intervene so directly in modern aircraft. The plane's control surfaces (flaps, ailerons, etc.) automatically adjust when an attitude deviation is indicated. Tellingly, this is called "slaving" the controls to the gyroscope. This leaves the pilot free to navigate in the macro sense of deciding where the plane should go and what is the best way to get there. The pilot has increased freedom as a result of making his second-to-second activities slaved to the device inside his plane that points upward.
Maybe this is akin to what is meant by "freedom in Jesus". There are places our Lord wants to go with us, but we'll never get there if we spend all of our time dealing with the storms and temptations of life, just trying to keep from crashing. We need to slave our habits and attitudes, the things we do on automatic, to Jesus. Fortunately, there is a Spirit living inside us who always knows which way is up, even in the midst of the storms. At first, we have to pay close attention to the Spirit's nudging while we try to maintain control ourselves. When we finally submit everything to Him, we find that we are not made completely powerless. Instead, we are free for the first time to really fly.
