Gravitational Force

Gravitation is a natural phenomenon by which objects with mass attract one another.[1]
In everyday life, gravitation is most commonly thought of as the agency which lends
weight to objects with mass. Gravitation compels dispersed matter to coalesce, thus it
accounts for the very existence of the Earth, the Sun, and most of the macroscopic objects
in the universe.

Modern physics describes gravitation using the general theory of relativity. Newton's law
of universal gravitation provides an excellent approximation for most calculations.

The terms gravitation and gravity are mostly interchangeable in everyday use, but a
distinction may be made in scientific usage. "Gravitation" is a general term describing the
phenomenon responsible for keeping the Earth and the other planets in their orbits around
the Sun; for keeping the Moon in its orbit around the Earth, for the formation of tides; for
convection (by which hot fluids rise); for heating the interiors of forming stars and
planets to very high temperatures; and for various other phenomena that we observe.
"Gravity", on the other hand, is described as the theoretical force responsible for the
apparent attraction between a mass and the Earth.[2] In general relativity, gravitation is
defined as the curvature of spacetime which governs the motion of inertial objects.

Since the gravitational force is experienced by all matter in the universe, from the largest
galaxies down to the smallest particles, it is often called universal gravitation. (Based
upon observations of distant supernovas around the turn of the 21st cent., a repulsive
force, termed dark energy dark energy, repulsive force that opposes the self-
attraction of matter (see gravitation ) and causes the expansion of the universe to
accelerate. The search for dark energy was triggered by the discovery (1998) in
images from the Hubble Space Telescope of a distant
Sir Isaac Newton Newton, Sir Isaac, 1642–1727, English mathematician and natural
philosopher (physicist), who is considered by many the greatest scientist that ever
lived.
was the first to fully recognize that the force holding any object to the earth is the same as
the force holding the moon, the planets, and other heavenly bodies in their orbits.
According to Newton's law of universal gravitation, the force between any two bodies is
directly proportional to the product of their masses (see mass mass, in physics, the
quantity of matter in a body regardless of its volume or of any forces acting on it.
The term should not be confused with weight , which is the measure of the force of
gravity (see gravitation ) acting on a body.
And inversely proportional to the square of the distance between them. The constant of
proportionality in this law is known as the gravitational constant; it is usually represented
by the symbol G and has the value 6.670 × 10−11 N-m2/kg2 in the meter-kilogram-second
(mks) system of units. Very accurate early measurements of the value of G were made by
Henry Cavendish.
Electromagnetic force

In physics, the electromagnetic force is the force that the electromagnetic field exerts on
electrically charged particles. It is the electromagnetic force that holds electrons and
protons together in atoms, and which hold atoms together to make molecules. The
electromagnetic force operates via the exchange of messenger particles called photons
and virtual photons. The exchange of messenger particles between bodies acts to create
the perceptual force whereby instead of just pushing or pulling particles apart, the
exchange changes the character of the particles that swap them.

Originally, electricity and magnetism were thought of as two separate forces. This view
changed, however, with the publication of James Clerk Maxwell's 1873 Treatise on
Electricity and Magnetism in which the interactions of positive and negative charges
were shown to be regulated by one force. There are four main effects resulting from these
interactions, which have been clearly demonstrated by experiment:

1. Electric charges attract or repel one another with a force inversely proportional to
the square of the distance between them: unlike charges attract, like ones repel.
2. Magnetic poles (or states of polarization at individual points) attract or repel one
another in a similar way and always come in pairs: every north pole is yoked to a
south pole.
3. An electric current in a wire creates a circular magnetic field around the wire, its
direction depending on that of the current.
4. A current is induced in a loop of wire when it is moved towards or away from a
magnetic field, or a magnet is moved towards or away from it, the direction of
current depending on that of the movement.

It is not the electromagnetic force but rather the strong nuclear force that holds together
the nucleus of an atom
Nuclear force

The nuclear force (or nucleon-nucleon interaction or residual strong force) is the force
between two or more nucleons. It is responsible for binding of protons and neutrons into
atomic nuclei. To a large extent, this force can be understood in terms of the exchange of
virtual light mesons, such as the pions. Sometimes the nuclear force is called the residual
strong force, in contrast to the strong interactions which are now understood to arise from
quantum chromodynamics (QCD). This phrasing arose during the 1970s when QCD was
being established. Before that time, the strong nuclear force referred to the inter-nucleon
potential. After the verification of the quark model, strong interaction has come to mean
QCD.

Since nucleons have no color charge, the nuclear force does not directly involve the force
carriers of quantum chromodynamics, the gluons. However, just as electrically neutral
atoms (each composed of cancelling charges) attract each other via the second-order
effects of electrical polarization, via the van der Waals forces (London forces), so by
analogy, "color-neutral" nucleons may attract each other by a type of polarization which
allows some basically gluon-mediated effects to be carried from one color-neutral
nucleon to another, via the virtual mesons which transmit the forces, and which
themselves are held together by virtual gluons. It is this van der Waals-like nature which
is responsible for the term "residual" in the term "residual strong force." The basic idea is
that while the nucleons are "color-neutral," just as atoms are "charge-neutral," in both
cases, polarization effects acting between near-by neutral particles allow a "residual"
charge effect to cause net charge-mediated attraction between uncharged species,
although it is necessarily of a much weaker and less direct nature than the basic forces
which act internally within the particles.

The nuclear force has been at the heart of nuclear physics ever since the field was born in
1932 with the discovery of the neutron by James Chadwick. The traditional goal of
nuclear physics is to understand the properties of atomic nuclei in terms of the 'bare'
interaction between pairs of nucleons, or nucleon-nucleon forces (NN forces).
In 1935, Hideki Yukawa made the earliest attempt to explain the nature of the nuclear
force. According to his theory, massive bosons (mesons) mediate the interaction between
two nucleons. Although, in light of QCD, meson theory is no longer perceived as
fundamental, the meson-exchange concept (where hadrons are treated as elementary
particles) continues to represent the best working model for a quantitative NN potential.

Historically, it was a formidable task to describe the nuclear force phenomenologically,

and the first semi-empirical quantitative models came in the mid-1950s. There has been
substantial progress in experiment and theory related to the nuclear force. Most basic
questions were settled in the 1960s and 1970s. In recent years, experimenters have
concentrated on the subtleties of the nuclear force, such as its charge dependence, the
precise value of the πNN coupling constant, improved phase shift analysis, high-precision
NN data, high-precision NN potentials, NN scattering at intermediate and high energies,
and attempts to derive the nuclear force from QCD.
Weak interaction

The weak interaction (often called the weak force or sometimes the weak nuclear
force[1]) is one of the four fundamental interactions of nature. In the Standard Model of
particle physics, it is due to the exchange of the heavy W and Z bosons. Its most familiar
effect is beta decay (of electrons outside atomic nuclei) and the associated radioactivity.
The word weak derives from the fact that the typical field strength is 1 / 1011 the strength
of the electromagnetic force and some 1 / 1013 that of the strong force, when forces are
compared between particles interacting with more than one way.

The weak interaction affects all left-handed leptons and quarks. It is the only force
affecting neutrinos (except for gravitation, which is negligible on laboratory scales). The
weak interaction is unique in a number of respects:

1. It is the only interaction capable of changing flavour.

2. It is the only interaction which violates parity symmetry P (because it almost
exclusively acts on left-handed particles). It is also the only one which violates
CP (CP Symmetry).
3. It is mediated by heavy gauge bosons. This unusual feature is explained in the
Standard Model by the Higgs mechanism.

Due to the large mass of the weak interaction's carrier particles (about 90 GeV/c2), their
mean life is about 3×10−27 seconds. Even at the speed of light this effectively limits the
range of the weak interaction to 10−18 meters, about 1000 times smaller than the diameter
of an atomic nucleus.

The Feynman diagram for beta-minus decay of a neutron into a proton, electron, and
electron antineutrino via an intermediate heavy W- boson

Since the weak interaction is both very weak and very short range, its most noticeable
effect is due to its other unique feature: flavour changing. Consider a neutron (quark
content: UDD, or one up quark and two down quarks).
Although the neutron is heavier than its sister nucleon, the proton (quark content UUD),
it cannot decay into a proton without changing the flavour of one of its down quarks.
Neither the strong interaction nor electromagnetism allow flavour changing, so this must
proceed by weak decay. In this process, a down quark in the neutron changes into an up
quark by emitting a W− boson, which then breaks up into a high-energy electron and an
electron antineutrino. Since high-energy electrons are beta radiation, this is called a beta
decay.

Due to the weakness of the weak interaction, weak decays are much slower than strong or
electromagnetic decays. For example, an electromagnetically decaying neutral pion has a
life of about 10−16 seconds; a weakly decaying charged pion lives about 10−8 seconds, a
hundred million times longer. A free neutron lives about 15 minutes, making it the
unstable subatomic particle with the longest known mean life.
Friction

Friction is the force resisting the relative motion of two surfaces in contact or a surface
in contact with a fluid (e.g. air on an aircraft or water in a pipe). It is not a fundamental
force, as it is derived from electromagnetic forces between atoms and electrons, and so
cannot be calculated from first principles, but instead must be found empirically. When
contacting surfaces move relative to each other, the friction between the two objects
converts kinetic energy into thermal energy, or heat. Friction between solid objects is
often referred to as dry friction or sliding friction and between a solid and a gas or liquid
as fluid friction. Both of these types of friction are called kinetic friction. Contrary to
many popular explanations, sliding friction is caused not by surface roughness but by
chemical bonding between the surfaces.[1] Surface roughness and contact area, however,
do affect sliding friction for micro- and nano-scale objects where surface area forces
dominate inertial forces.[2] Internal friction is the motion-resisting force between the
surfaces of the particles making up the substance. Friction should not be confused with
traction. Surface area does not affect friction significantly because as contact area
increases, force per unit area decreases. However, in traction surface area is essential.
Applied Force

An applied force is a force which is applied to an object by a person or another object. If

a person is pushing a desk across the room, then there is an applied force acting upon the
object. The applied force is the force exerted on the desk by the person.

If you put energy into an object, then you do work on that object.

If a first object is the agent that gives energy to a second object, then the first
object does work on the second object. The energy goes from the first object
into the second object. At first we will say that if an object is standing still, and
you get it moving, then you have put energy into that object.

For example, a golfer uses a club and gets a stationary golf ball moving when
he or she hits the ball. The club does work on the golf ball as it strikes the ball.
Energy leaves the club and enters the ball. This is a transfer of energy. Thus,
we say that the club did work on the ball.

And, before the ball was struck, the golfer did work on the club. The club was
initially standing still, and the golfer got it moving when he or she swung the
club.

So, the golfer does work on the club, transferring energy into the club, making
it move. The club does work on the ball, transferring energy into the ball,
getting it moving.
Air Resistance Force

The air resistance is a special type of frictional force which acts upon objects as they
travel through the air. The force of air resistance is often observed to oppose the motion
of an object. This force will frequently be neglected due to its negligible magnitude (and
due to the fact that it is mathematically difficult to predict its value). It is most noticeable
for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects
with large surface areas.

As an object falls through air, it usually encounters some degree of air resistance. Air
resistance is the result of collisions of the object's leading surface with air molecules. The
actual amount of air resistance encountered by the object is dependent upon a variety of
factors. To keep the topic simple, it can be said that the two most common factors which
have a direct affect upon the amount of air resistance are the speed of the object and the
cross-sectional area of the object. Increased speeds result in an increased amount of air
resistance. Increased cross-sectional areas result in an increased amount of air resistance.
Tension Force

The tension force is the force which is transmitted through a string, rope,
cable or wire when it is pulled tight by forces acting from opposite ends. The
tension force is directed along the length of the wire and pulls equally on the
objects on the opposite ends of the wire.

In physics String Tension is the magnitude of the pulling force exerted by a

string, cable, chain, or similar object on another object. Tension is measured
Newtons (kgm/s2) and is always parallel to the string on which it applies.
There are two basic possibilities for systems of objects held by strings.[1]
Either acceleration is zero and the system is therefore in equilibrium or there
is acceleration and therefore a net force is present. Note that a string is
assumed to have negligible mass.
Elastic force

In engineering, iso-elastic refers to a system of elastic and tensile parts (springs and
pulleys) which are arranged in a configuration which serves to isolate physical motion at
one end from affecting the same motion at the other end. Such a device must be able to
maintain angular direction and load bearing over a large range of motion.

The most prominent use of an iso-elastic system is in the supporting armature of a

Steadicam. The arm of a Steadicam is used to isolate a film or video camera from the
operator's movements. Steadicam arms all work in a fashion similar to a spring lamp;
each arm has 2 sections (similar to and labelled like a human arm), both the upper and
fore-arm sections consist of a parallelogram with diagonal iso-elastic cable-pulley-spring
system. The iso-elastic system is tensioned carefully to counteract the weight of the
camera and steadicam sled. This allows the camera and operator to move vertically,
independently of each other. For example as the operator runs, the bouncing of his body
is absorbed by the springs, keeping the camera steady. The arm also has unsprung hinges
at both ends of each arm allowing it to bend in the horizontal plane (just like your elbow,
not like a spring lamp).

To understand how an iso-elastic system works, we must first understand how springs
work. The tension (elastic force) in a spring is proportional to its extension according to
Hooke's law. This means that if a weight is hung on a spring it will oscillate with simple
harmonic motion. This is because when the weight is above the balance point, the spring's
tension is reduced so the weight falls due to gravity, and when the weight is below the
balance point the spring's tension will pull it back upwards.

If a simple spring system were used in a steadicam, then if the operator moved vertically,
the camera would execute simple harmonic motion, and bounce up and down. Instead, an
iso-elastic system is employed.

The springs used are large, stiff springs with a high modulus of elasticity, and they are
highly tensioned. A compound pulley system is then used so that the large force exerted
by the spring can be divided by a factor of (say) five. The cable exiting the pulley system
will have a moderate tension on it, but most importantly, when the cable is drawn in or
out, and the extension of the spring changes by only a fifth of that distance, so that the
tension force of the spring will not change much. The result is that the spring-pulley
system can produce a fairly constant tension in the cable over a large range of movement.
The almost constant force exerted by an iso-elastic system is employed in the armature of
a steadicam, to counteract the constant force of gravity on the camera's and mount's mass.
The result is that the weight of the camera is exactly balanced (well, nearly) by the
tension force throughout the entire range of vertical movement, so even when the
operator jumps vertically, the camera will retain its vertical position due to inertia, but
remain balanced, just with the armature at a different angle.

As a result the camera doesn't bounce up to the 'balanced' position after a move, for
example when the operator steps up onto a curb from the road. This allows the camera to
be more isolated and independent of the operator's moves. The operator can of course
deliberately move the camera up or down, if desired.
Normal force

n physics, the normal force (or in some books N) is the component, perpendicular to
the surface of contact, of the contact force exerted by, for example, the surface of a floor
or wall, on an object, preventing the object from entering the floor or wall. In a static
situation it is just enough to balance the force with which the object pushes, e.g. its
weight on the floor, or a smaller force if somebody leans against a wall. If an object hits
the surface with some speed, the normal force provides for a rapid negative acceleration,
depending on how flexible the floor/wall is (and, of course, if it can provide enough force
for stopping the object instead of breaking). Also, if the object is soft, only the outer part
needs to decelerate rapidly, the inner part can do that more gradually, while the layer in
between is compressed.

In general, the magnitude of the normal force is the projection of the surface traction, T,
in the normal direction, n, and so the normal force vector can be found by scaling the
normal direction by that force. The surface traction, in turn, is equal to the dot product of
the unit normal with the stress tensor describing the stress state of the surface. That is,

Or, in indicial notation,