Einstein also derived his most famous equation from the principles of relativity. Mass and
energy can be converted into one another. An object with a rest mass of can be
converted into an amount of energy, given by:
We will put this equation to work when we look at nuclear physics.
Relativity and Graphs
One of the most common ways SAT II Physics tests your knowledge of special relativity is
by using graphs. The key to remember is that, if there is a dotted line representing the
speed of light, nothing can cross that line. For instance, here are two graphs of kinetic
energy vs. velocity: the first deals with normal speeds and the second deals with
relativistic speeds:
In the first graph, we get a perfect parabola. The second graph begins as a parabola, but
as it approaches the dotted line representing c, it bends so that it constantly approaches c
but never quite touches it, much like a y = 1/x graph will constantly approach the x-axis
but never quite touch it.
The Discovery of the Atom
The idea that matter is made up of infinitely small, absolutely simple, indivisible pieces is
hardly new. The Greek thinkers Leucippus and Democritus suggested the idea a good 100
years before Aristotle declared it was nonsense. However, the idea has only carried
scientific weight for the past 200 years, and it only really took off in the past century.
Thompson’s “Plum Pudding” Model
The first major discovery that set off modern atomic theory was that atoms aren’t in fact
the smallest things that exist. J. J. Thompson discovered the electron in 1897, which led
him to posit a “plum pudding” model (a.k.a. the “raisin pudding” model) for the atom.
Electrons are small negative charges, and Thompson suggested that these negative
charges are distributed about a positively charged medium like plums in a plum pudding.
The negatively charged electrons would balance out the positively charged medium so
that each atom would be of neutral charge.