knowledge, science, and the universe chapter 1 - Physical Science ...

SECTION 1–4 The Universe Around Us 9
figure 1.10
A drawing of the complex
Ptolemaic model and the
simpler Copernican model.
Only one of the competing
models of the solar system can
be true. The simpler
Copernican model is correct.
Ptolemaic Model
As measurements of planetary positions
became more precise, the Ptolemaic model was
changed to fit the data by including more circular
paths for the planets to move on. Soon
the long- accepted Ptolemaic model became an
entangled collection of circles connected to
circles connected to even more circles that was
complex beyond reason. Nicholas Copernicus
saw that if the sun were at the center of the solar
system and Earth moved around it, the complex
arrangement of circles could be discarded
in favor of a simple, single orbital path for each
planet.
The simplicity of this new scheme correctly
guided mankind to the truth. The incorrect
assumption that the Sun and planets orbited
about Earth was replaced by the correct and
much simpler assumption that Earth and planets
orbited about the Sun.
1–4 ThE uNIVERSE AROuNd uS
Copernican Model
objects nearby, or large bodies halfway across
the universe, all things are made of matter and
energy organized differently and rather uniquely
on different lengths and scales. This organization
is governed by the influence of interactions
that give rise to forces.
We know of four interactions in nature:
the strong nuclear, the electromagnetic, the
weak nuclear, and gravity. Forces are felt by
matter in accordance with attributes possessed
by the matter. The nuclear forces act on nucleons
and quarks, the electromagnetic force acts
on charged particles, and gravity acts on mass.
In some structures these four interactions may
be at work simultaneously with similar or
opposing effects.
The relative strengths of these interactions
are surprisingly diverse. If two protons are
placed side by side and the forces on them are
measured, the strong nuclear force acting on
the protons is 100 times greater than the electromagnetic
force, 10 13 times greater than the
Interaction
Any of four fundamental
ways in which elementary
particles and bodies can
influence each other.
force
A push or pull on an object.
Strong Nuclear
Interaction
The interaction between
nucleons that gives rise to
the strong force.
Electromagnetic
Interaction
The interaction between
charged objects that gives
rise to the electromagnetic
force.
weak Nuclear
Interaction
The interaction between
nucleons that gives rise to
the weak force.
Gravity
The interaction between
anything with mass that
gives rise to the gravitational
force.
We close this chapter with a brief tour of
the Universe. On this tour, we pay attention
to the sizes of objects and to the forces that govern
their interactions. This section may be
viewed as an outline of the rest of the book. In
later chapters, we look at the specific laws and
forces that govern objects of different sizes, and
try to understand how these interactions produce
the beauty and order of the world around
us.
Interactions and Force
Whether we are looking at microscopic
Table 1.1 — The four Interactions of Nature
Relative
force Strength Range Acts upon
Nuclear Strong 10 38 Atomic Nuclei 10 -15 m Nucleons
Electromagnetic 10 36 Earth Diameter 10 7 m Charged Matter
Nuclear Weak 10 25 Atomic Nuclei 10 -15 m Nucleons
Gravity 1 Entire Universe 10 26 m Mass
Force strength varies with distance. The relative strength listed here is
what is felt by two protons sitting side-by-side (approximately 10 -15 m
apart).