Newton’s Universe
CHAPTER 5
5.1 The Idea of Gravity: The Apple and the
Moon
 An apple falling towards earth is accelerated by Earth’s
gravitational pull. The directions of the apple’s velocity,
acceleration, and gravitational force are all downward towards
Earth’s center.
 The moon’s motion is different in that the direction of its velocity
is parallel to Earth’s surface rather than towards its center.
According to inertial view, forces cause accelerations, not
velocities, so the forces on the apple and the moon could be
similar despite the dissimilarity of the velocities.
 Inertia states that in order for the moon to deviate from straightline motion a force must act on it. To put it in perspective, if the
moon were at point A and if no force acted on it, it would move in
a straight line toward point B. Instead it moves around to point
C. The force required to move the moon to point C from point A
is directed towards Earth’s center, just like the force on the
falling apple.
5.1 Cont.
 Earth’s gravitational attraction is the force that acted on both the
moon and the apple. Newton’s observation of the apple and the
moon brought about the law of motion which tells us that the
moon’s acceleration must be inward just like the apple.
 Newton argued that if you throw an object at a high enough
speed that the curvature of its path just matches the Earth’s
curvature causing the object to go into orbit. The required speed
to do this is about 8 km/s. The less the gravitational pull the less
the required speed will be. The moon’s speed is only 1 km/s.
5.2 The Law of Gravity: Moving the
Farthest Star
 Small astronomical objects are held in their orbit by the
gravitational pull exerted on them by larger astronomical objects.
The solar system orbits in ellipses (elongate circles) because of
the law of inertia: the sun’s gravitation pull bends their orbits
into ellipses. Due to the law of force pairs, every object will pull
back on the larger object.
 Newton reasoned that gravitational force is universal and all
objects exert a force on each other and depending on the objects
mass, the gravitational attraction will be decided. The
gravitational force will also get smaller when the distance
between the objects gets larger.
5.2 Cont.
 When referring to the distance between two objects, we are
referencing the distance from the center of the objects. When
measuring the distance from Earth you must always include 1
Earth radii from the center of the Earth plus any Earth radii
above the earth.
 If you double Earth’s mass this would double your weight and
halving Earth’s mass would halve your weight. If you tripled
your mass while simultaneously doubling Earth’s mass, your
weight would be multiplied by six since the force is proportional
to the product of the two masses (Your mass – 3 X Earth’s mass
– 2 would equal 6).
 Earth’s mass is 100 times the moon’s mass and the Earth’s radius
is 4 times the moon’s radius. Using the law of gravity you would
get that your weight is 6 times larger on Earth than on the moon
(100/4squared=6).
5.3 Gravitational Collapse: The Evolution
of the Solar System
 When gaseous hydrogen atoms densely gather together into
great gas clouds, forming a place in the center, it creates a
great gravitational attraction. This gravitational attraction
can become a new star if enough material falls into this center.
The falling together of this matter is referred to as a
gravitational collapse. This process is how our solar system
was created with the sun being created in the center and the
left over material that separated from the spin of the center
formed the planets.
 The sun was circling so fast during its creation that the speed
began to build intense heat creating nuclear fusion. Nuclear
fusion occurs when violently colliding nuclei come together
and create a larger single nucleus. The intense heat created
during the nuclear fusion stopped the ball of gas from
collapsing down any further.
5.3 Cont.
 As each star has a birth it also has a death. Once hydrogen fuel is
depleted nuclear fusion will cease at the hydrogen level and
commence at the helium stage. After helium nuclear fusion has
depleted its energy source it will begin to be overtaken by gravity
again. Without a nuclear heating source there will be nothing to
stop the star from collapsing in upon itself. After the collapse,
and all the nuclei and electrons are packed together, there will
only be a remnant of the dying star that will be similar to a dying
ember and this is known as a white dwarf.
5.4 Gravitational Collapse: The Deaths of
More Massive Stars
 Each collapse of a star depends upon its mass. White dwarfs occur
when a star weighs up to about 10 times the mass of the sun. Stars
that are at least 10 times as massive as the sun go through the event
of a supernova explosion. Supernovas occur during the greater
stages of nuclear fusion as it attempts to fuse iron and turns the
entire star into a massive iron core which then abruptly collapses
upon itself in just one second and blasts outwards.
 After a supernova only about 10 to 20% of the original star remains.
The inward pull of gravity on the remainder of the star that was as
massive as 10 to 30 suns will have a “neutron exchange force” that
acts between neutrons creating a neutron star.
 A star that has a mass greater than 30 suns goes through a dying
process known as a black hole. The ensuing collapse of a star this
massive has such a strong gravitational force that nothing can stop
it.
5.5 The Newtonian Worldview: A
Democratic, Mechanical Universe
 Newtonian worldview is known as the new philosophical and
religious view that was ushered in by the sun-centered
astronomy and inertial physics during the sixteenth and
seventeenth century.
 The new physics during this time was underlain by the focus on
the simplest and smallest components of matter known as the
atom. Atomism was the idea that nature can be reduced to the
motions of tiny material particles.
 Newtonian worldview saw the universe as the mechanical
universe. Physicist thought of the universe as a clockwork
mechanism whose operating principles were the laws of nature
and whose parts were atoms. Newtonian physics saw that every
physical system is entirely predictable.
5.6 Beyond Newton: Limitations of
Newtonian Physics
 During the eighteenth and nineteenth century experiments
that were tested, Newtonian physics stood up in quantitative
detail every time. But near the twentieth century, there were
tests that couldn’t be reconciled with Newtonian physics. The
confliction came during four extreme situations: high speeds,
enormous gravitational forces, huge distances, and tiny
distances.
 In order to get correct predictions at both high and low speeds
special relativity was born which gave correct predications
at all speeds. To combat the discrepancies with gravitational
force general relativity was developed which gives correct
predictions for all situations with regard to the theory of
gravity. Quantum physics helps with predictions for
objects of all sizes, from microscopic to macroscopic.