Physics 101- Introduction to Mechanics
April 10- Conservation of Momentum






  More conservation Laws!

The idea of finding a cosnerved quantity is extrememly useful- it can greatly simplify our calculations as well as provide some insight into how the unvierse works.

As you might guess there are other conservation laws beyond energy and momentum.

  1. Energy- Recall that along with the traditional forms of energy (kinetic, gravitational, light, electrical, chemical, etc.) there is also the energy associated with mass.
  2. Momentum- This has been shown to be conserved even on the subatomic scale.
  3. Angular momentum- We'll study this next week as part of this unit.  Not only does it include macroscopic objects that spin, but also the particle spin or intrinsic angular momentum found in subatomic particles.
  4. Electric charge- if the system is positively charged in the beginning, it must be positively charged in the end (we do not get to create any net charge- we can create a pair of particles, one negatively charged and the other equally positively charged)
  5. Color charge- This behaves much like electrical charge, but instead quarks can come in three, not two varieites- red, green or blue (don't attach too much significance to the names, they're just names)
  6. Quark number- how ever many (quarks - anti-quarks) we have in the beginning, we much have the same at the end.
  7. Lepton number- Electrons are the most common lepton, but there are also muons and taus which are very similar to electrons, but are more massive.  Also, there are small, nearly massless particles know as neutrinos within this family of particles.  This counts not only electrons in the interaction, but also positrons (anti-electrons) and electron neutrinos (and anti-electron neutrinos).
All of these conservation laws are consequences of the Standard Model of particle interactions.  As you can see the conservation of energy, momentum and angular momentum are very fundamental to the study of physics.  As far as we know, these seven quantities are the only ones that are always conserved.

Like most "laws" in science, we can never really know if they are true, only if they are proven to be incorrect.  Perhaps there is a very rare process that we have not yet observed that may not respect these conservation laws. If such processes are found, we will have to downgrade at least one of these laws to an approximate conservation law. 

For example, it was only a few years ago that we learned that the various neutrinos have mass.  Because of that we now know that the electron, muon or tau numbers don't have to be conserved individually, rather the total lepton number is conserved. 

Similarly, some quantities are conserved in all but a few unusually situations.  Some of these are strangeness, topness and bottomness.  (The names come from the various flavors of quarks.)

There is a page within our web site that describes more about the four fundamental forces and some of the particles that are related to these forces.

Places on the internet ot learn more about these new conservation laws:

  • The Science of matter, space and time at FermiLab.  This is a great place to start your tour of "particle physics".  It offers a qualitative overview of what the world is made of and what the standard model is.  They even have a short video that gives an overview of the standard model.
  • Particle Adventure- This is a great site that explains the physics behind quarks, electrons and other fundamental particles.  (A bit more technical than the previous site.)
  • Stanford Linear Accelearator Center (SLAC) Virtual Visitor Center- This site not only has information on the fundamental physics, it also describes the methods that physicists use to study the physics of small particles.


About the Course
Study Hints
Problem Solving
Homework- assignments & solutions
Miscellaneous links
About Dr. Jeff



Jeff Phillips
Loyola Marymount University
Spring 2002