Rotation
Contents
Applications
Worked examples
© The scientific sentence. 2010
Formulas
θ = (1/2) α t2 + ωot + θo
ω = α t + ωo
ω2 - ωo2 = 2 α (θ - θo)
ar = ω2(t) r
at = α r
Circ. Unif. motion:
1 rev = 2π rad
θ = ωt
v = ω r
T = 2π/ω = 1/ƒ
ar = ω2r = v2/r
at = 0
|
| Physics laws
Breaking a Physics law
In Geometry, a postulate is a truth that we don't know proving; and we admit it as true in order to support the continuation of a reasoning. For example, the statement "The line is the shortest path between two points.". In Physics, a postulate has the same meaning. For example
"Even though is accelerated, an electron orbiting the nucleus does not emit radiation unless
it transits to a lower orbit". Therefore we need to accept without proof this statement to be able to go further and derive other results for the behavior on an electron within an atom. Newton's laws
serve also as postulates.
The law in Physics is similar to a postulate, but it is formulated.
Newton's laws are an example. If we consider Newton's second law which
states "The net force exerted on a system is equal to its inertial mass times its
acceleration"; the vectorial formula for this statement is Fnet = m a.
A law in Physics remains valid in a certain framework. Recall that Newton's third
law is valid only in an inertial frame of reference. Notice that, as a measurement, a law
is always approximative. In addition, in Physics, or in Science, a law is open to change.
Who violate a law in Physics is very rewarding, at the difference
to who break a criminal or civil law.
A postulate in Mathematics is not open to discussion and not subject to validation. It cannot be tested because Mathematics is a pure abstraction. In Physics, there are often means to
prove or disprove and test the truth of a physical statement. The rule is "a law must be
in accord with experiment". So a law in Physics in not immutable. It must comply with newly
discovered phenomena.
Some results come from experiment and constitute an empirical law. For example
Moseley's law. This law is not a statement or a principle to whom the truth
will be tested. It is already done. It cannot be abandoned, but just could be
modified to include new effect. An empirical law is a rule that describe an experiment
over a limited range of conditions.
A law remains active while it works.
|
|
