Hobbies And Interests
Gyroscope
A gyroscope is a spinning object that is symmetrical about its axis of rotation. A basketball on a finger, a pencil rolling on a table, a motorcycle tire --- these are all gyroscopes. Newton's laws of motion apply to every little part of a gyroscope, so the motion could be calculated by observing how the momentum of each tiny element of the gyroscope is changed by the force acting upon it. But there's a far easier shortcut to find equations of motion that apply to the whole spinning body at once. A non-spinning object will change its momentum when a force is applied. A spinning object will change its angular momentum when a torque is applied.
A Spinning Top
Gyroscopic motion is a special case of rigid body motion -- a topic that would take some time to explore -- but there is an easy way to introduce yourself to some of the peculiarities of gyroscopic motion. If you place a toy top on its point, it immediately falls over. If you spin that same top, it will roam around the table for a while, before slowing down and falling. Gravity still acts on the top, but the spinning changes the top's response to the force of gravity.
Angular Momentum and Torque
A spinning object has an angular momentum that points along its axis. It will keep that angular momentum unchanged, unless a torque acts on it. A torque is a force applied at a distance from the axis of a spinning object. If the top's axis is perfectly vertical, gravity acts on the center of mass of the top, which is right on the axis. Thus there is no torque and the angular momentum will not change. Unchanged angular momentum means the spin axis doesn't move, so the top keeps spinning upright. When imperfections force the top to tip a little, gravity doesn't pull right down the axis anymore. But there's a funny thing about torque: it's at right angles to the force. So when a top starts to tip and gravity pulls on it, it doesn't pull the top over -- it causes it to start wobbling.
Conserving Angular Momentum
Newton's first law posits the idea that a body will keep the same momentum unless changed by another force.The same kind of thing is true for spinning bodies: a body will keep its angular momentum unless a torque changes it. So if you could isolate a spinning object from external forces that could exert a torque, then the object will keep its angular momentum in the same direction. That's how gyroscopic instruments work. A spinning section is enclosed in gimbals --- mechanical linkages that let the gyro spin freely --- and the spin keeps going in the same direction. So if a gimballed gyro starts out pointed at the Eastern horizon, it will stay pointed that same way regardless of the twists and turns on its frame.
Precession
Watching a vertically suspended, spinning bicycle tire react to gravitational attraction is one of the most compelling visual demonstrations of the effect of external force on a gyroscope. Videos of this are available on the Internet, at such places as the MIT open course on rolling motion and gyroscopes. If you attach a string to one end of the axle of a bicycle tire, and then let the tire drop, you would expect it to fall down and hang from the attachment point. But when the tire is spinning, the force also acts as a torque and, as mentioned above, instead of pulling down, it acts at right angles and sends the tire spinning around in midair. That motion is called precession, defined as the response of a gyroscope to an external force.