Physics 101- Introduction to Mechanics
February 18- Applications of Newton's Laws

Fundamental forces- see chapter 46 of our text (the portion on reserve) for more details (also, the Particle Adventure on the web is excellent)

There are only four forces- all of our categories (tension, friction, etc) are examples of these four.  In fact, most are simply electromagnetic force- force between electrons and atoms.

• Gravity acts on anything that has mass and is only important for large masses  (of the four it is the weakest)
• Electromagnetic acts on anything with charge
• The others, weak & strong, are important inside the atom
Historically, physicists have tried to reduce these four forces down to one.  In the 1800ís it was shown that electricity and magnetism are really the same thing.  In mid- 1900ís (~1970) it was shown that the weak and electromagnetic forces were really parts of one force- the electroweak.  Grand Unified Theories are working on the electroweak and strong combination

Electromagnetism
Here the quantity that matters is the charge (+ or -).  The force is governed by photons (aka light).

Gravity
Here what matters is the quantity of mass (no sign is attached).  The messenger particles, gravitons, have not been observed.

Strong
Here what matters is not charge or mass, but color!    For now the smallest particles, quarks, are the most fundamental building blocks.

There are six kinds of quarks (six flavors) up, down, strange, charm, truth and beauty.  (great, huh?).  Each of these can come in either red, blue or yellow (or anti-red, anti-blue, and anti-yellow).  Much like charge provides us with rules on how electricity behaves, color does the same for the strong force.

Quantum chromodynamics is the theory of the strong interaction.  Like charge, color is conserved in interactions.  Also, particles are color neutral.

Oh, yeah, the messenger particles that help to hold the quarks together?  Theyíre called gulons.

Weak
The odd thing here is the characteristics of the messenger particles.  For EM and gravity they are massless, here they are very heavy (nearly 100 times heavier than the proton).

The weak force is felt by electrons (and similar particles) and neutrinos.  These are just absolutely fun guys.  They have nearly no mass, and hardly interact with anything (thus the name weak!).  You have thousands of them passing through your body right now due to the nuclear reactions in the sun.

All of this is known as the standard model among physicists.  There is much experimental data to support it.  So far nothing disproves it.

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 Jeff Phillips phillips@lmu.edu Loyola Marymount University Spring 2002