©Timothy E. Chupp, 1995
Up to this point, we have discussed the motion of objects without thinking about an object's structure. With the exception of the priciple of the center of gravity of an object, our considerations have not included the smaller pieces that make up the object and the distribution of those pieces. In fact it should be no surprise that an object is made up of atoms and molecules, which will be discussed later. It is also true that the distirbution of mass within an object can lead to different parts of the object moving in different ways. For example, consider a bar-bell, an object made up of two massive balls with a light rod connecting them. The entire bar-bell can be in motion in a way that each of these three pieces has the same velocity and any given time. Another scenario is the two balls moving in opposite directions with velocities of equal magnitude. In this case, the center of mass of the bar bell is at rest, but the pieces are not. They are rotating.
Our task now is incorporating this rotation into our description of motion. For the most part, our apporach merely takes the distribution of mass of an object into account by adding up the contributions of the small masses that make up the body. However we will find a new and meaningful physical quantity: angular momentum. Angular momentum, like linear momentum and energy, is a conserved physical quantity. Angular momentum plays an extremely important role in physics at the atomic and sub-atomic scale. Many elementary particles, including the electron, proton and neutron, have intrinsic angular momentum. The structure and stability of atoms leading to the periodic table, the building of atoms into molecules, and the binding of neutrons and protons in nuclei follows completely from manifestations of this intrinsic angular momentum, called spin. Conservation of angular momentum also leads to the observabel effects used in Nuclear Magnetic Resonance and Magnetic Resonance Imaging.
Fig 1 A barbell with moving without rotating and rotating with no net momentum of its center of mass.