Telephone Bells - The old Type

Where a bell is powered by AC a different design, the polarised bell, may be used. These have an armature containing a permanent magnet, so that this is alternately attracted and repelled by each half-phase and different polarity of the supply. In practice, the armature is arranged symmetrically with two poles of opposite polarity facing each end of the coil, so that each may be attracted in turn. No contact breaker is required, so the bells are reliable for long service. For this reason they were widely used for telephone bells.

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Electric Bell

An Electric bell works on the idea of Electromagnetism.


Working: The bell or gong (B), which is often in the shape of a cup or half-sphere, is struck by a spring-loaded arm with a metal ball on the end called a clapper (A), actuated by an electromagnet (E). In its rest position the clapper is held away from the bell a short distance by its springy arm. When an electric current is passed through the winding of the electromagnet it creates a magnetic field that attracts the iron arm of the clapper, pulling it over to give the bell a tap. This opens a pair of electrical contacts (T) attached to the clapper arm, interrupting the current to the electromagnet. The magnetic field of the electromagnet collapses, and the clapper springs away from the bell. This closes the contacts again, allowing the current to flow to the electromagnet again, so the magnet pulls the clapper over to strike the bell again. This cycle repeats rapidly, many times per second, resulting in a continuous ringing. 

Another type, the single-stroke bell, has no interrupting contacts. The hammer strikes the gong once each time the circuit is closed.

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Electromagnetism

History of the Electromagnetic Theory:
Electricity and Magnetism which were once thought of two different types of interactions but was changed when scientists, especially Maxwell observed that there were a few things common between Electricity and Magnetism. These were:

James Clerk Maxwell

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 (clockwise or counter-clockwise) 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.

While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation. As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.

Hans Christian Ørsted

The Electromagnetic Force: The electromagnetic force is one of the four known fundamental forces. The other fundamental forces are: the strong nuclear force, which binds quarks to form nucleons, and binds nucleons to form nuclei, the weak nuclear force, which causes certain forms of radioactive decay, and the gravitational force. All other forces are ultimately derived from these fundamental forces and momentum carried by the movement of particles.

The electromagnetic force is the one responsible for practically all the phenomena one encounters in daily life above the nuclear scale, with the exception of gravity. Roughly speaking, all the forces involved in interactions between atoms can be explained by the electromagnetic force acting on the electrically charged atomic nuclei and electrons inside and around the atoms, together with how these particles carry momentum by their movement. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which come from the intermolecular forces between the individual molecules in our bodies and those in the objects. It also includes all forms of chemical phenomena.

A necessary part of understanding the intra-atomic to intermolecular forces is the effective force generated by the momentum of the electrons' movement, and that electrons move between interacting atoms, carrying momentum with them. As a collection of electrons becomes more confined, their minimum momentum necessarily increases due to the Pauli exclusion principle. The behaviour of matter at the molecular scale including its density is determined by the balance between the electromagnetic force and the force generated by the exchange of momentum carried by the electrons themselves.

Uses Of Electromagnetism:

Fire Alarm

1. To make temporary powerful Electromagnets
2. used in electric bells
3. used in fire alarms
4. used in telephones
5. used in motors

Electric Bell

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Electric Field and the concept of Electromagnetism

Micheal Faraday first introduced the idea of electric fields and electromagnetism. He to coil of copper and attached an Ammeter to it. Then he put a magnet thorough the coil and observed a flow of electricity. The he pulled the magnet out and observed that there was no electricity.

An electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding.


An electric field that changes with time, such as due to the motion of charged particles in the field, influences the local magnetic field. That is, the electric and magnetic fields are not completely separate phenomena; what one observer perceives as an electric field, another observer in a different frame of reference perceives as a mixture of electric and magnetic fields. For this reason, one speaks of electromagnetism or electromagnetic fields.

The electric field intensity is defined as the force per unit positive charge that would be experienced by a stationary point charge, or "test charge", at a given location in the field:

where

F is the electric force experienced by the test particle

q_t is the charge of the test particle in the electric field

E is the electric field wherein the particle is located.

Taken literally, this equation only defines the electric field at a specific location as the force experienced by a stationary test charge at that point (with the sign of q_t, positive or negative, determining the direction of the force). Given that electric fields are generated by electrically charged particles, adding or moving a source charge, q_s, will alter the electric field distribution. Therefore, it is important to remember that an electric field is defined with respect to a particular configuration of source charges. In practice, this is achieved by placing test particles with successively smaller electric charge in the vicinity of the source distribution and measuring the force exerted on the test charges as their charge approaches zero.

This allows the electric field to be determined from the distribution of its source charges alone.

As is clear from the definition, the direction of the electric field is the same as the direction of the force it would exert on a positively-charged particle, and opposite the direction of the force on a negatively-charged particle. Since like charges repel and opposites attract (as quantified below), the electric field tends to point away from positive charges and towards negative charges.

Based on Coulomb's law for interacting point charges, the contribution to the E-field at a point in space due to a single, discrete charge located at another point in space is given by the following:

where

Q is the charge of the particle creating the electric force,

r is the distance from the particle with charge Q to the E-field evaluation point,

 is the unit vector pointing from the particle with charge Q to the E-field evaluation point,

ε₀ is the electric constant.

The total E-field due to a quantity of point charges, n_q, is simply the superposition of the contribution of each individual point charge:

Alternatively, Gauss's law allows the E-field to be calculated in terms of a continuous distribution of charge density in space, ρ:

Coulomb's law is actually a special case of Gauss's Law, a more fundamental description of the relationship between the distribution of electric charge in space and the resulting electric field. Gauss's law is one of Maxwell's equations, a set of four laws governing electromagnetic.

Electromagnetism: Electromagnetism is the force that causes the interaction between electrically charged particles; the areas in which this happens are called electromagnetic fields. It is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation.

Electromagnetism is the interaction responsible for practically all the phenomena encountered in daily life, with the exception of gravity. Ordinary matter takes its form as a result of intermolecular forces between individual molecules in matter. Electrons are bound by electromagnetic wave mechanics into orbitals around atomic nuclei to form atoms, which are the building blocks of molecules. This governs the processes involved in chemistry, which arise from interactions between the electrons of neighboring atoms, which are in turn determined by the interaction between electromagnetic force and the momentum of the electrons.

Electromagnetism manifests as both electric fields and magnetic fields. Both fields are simply different aspects of electromagnetism, and hence are intrinsically related. Thus, a changing electric field generates a magnetic field; conversely a changing magnetic field generates an electric field. This effect is called electromagnetic induction, and is the basis of operation for electrical generators, induction motors, and transformers. Mathematically speaking, magnetic fields and electric fields are convertible with relative motion as a 2nd-order tensor or bivector.

Electric fields are the cause of several common phenomena, such as electric potential (such as the voltage of a battery) and electric current(such as the flow of electricity through a flashlight). Magnetic fields are the cause of the force associated with magnets.

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Electricity

Electricity is the flow of electrons form one place to another. It is also the flow of electric charge.

Charge or Electric charge:
It is something a man can never define explain and may not also in the future. But it is only understood as a concept.

Electric charge is a property of certain subatomic particles, which gives rise to and interacts with the electromagnetic force, one of the four fundamental forces of nature. Charge originates in the atom, in which its most familiar carriers are the electron and proton. It is a conserved quantity, that is, the net charge within an isolated system will always remain constant regardless of any changes taking place within that system. Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire. The informal term static electricity refers to the net presence (or 'imbalance') of charge on a body, usually caused when dissimilar materials are rubbed together, transferring charge from one to the other.

The presence of charge gives rise to the electromagnetic force: charges exert a force on each other, an effect that was known, though not understood, in antiquity. A lightweight ball suspended from a string can be charged by touching it with a glass rod that has itself been charged by rubbing with a cloth. If a similar ball is charged by the same glass rod, it is found to repel the first: the charge acts to force the two balls apart. Two balls that are charged with a rubbed amber rod also repel each other. However, if one ball is charged by the glass rod, and the other by an amber rod, the two balls are found to attract each other. These phenomena were investigated in the late eighteenth century by Charles-Augustin de Coulomb, who deduced that charge manifests itself in two opposing forms. This discovery led to the well-known axiom:like-charged objects repel and opposite-charged objects attract.

The force acts on the charged particles themselves, hence charge has a tendency to spread itself as evenly as possible over a conducting surface. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by Coulomb's law, which relates the force to the product of the charges and has an inverse-square relation to the distance between them. The electromagnetic force is very strong, second only in strength to the strong interaction, but unlike that force it operates over all distances. In comparison with the much weaker gravitational force, the electromagnetic force pushing two electrons apart is 10⁴² times that of the gravitational attraction pulling them together.

The charge on electrons and protons is opposite in sign, hence an amount of charge may be expressed as being either negative or positive. By convention, the charge carried by electrons is deemed negative, and that by protons positive, a custom that originated with the work of Benjamin Franklin. The amount of charge is usually given the symbol Q and expressed in coulombs. Each electron carries the same charge of approximately −1.6022×10⁻¹⁹ coulomb. The proton has a charge that is equal and opposite, and thus +1.6022×10⁻¹⁹  coulomb. Charge is possessed not just by matter, but also by antimatter, each antiparticle bearing an equal and opposite charge to its corresponding particle.

Charge can be measured by a number of means, an early instrument being the gold-leaf electroscope, which although still in use for classroom demonstrations, has been superseded by the electronic electrometer.

Electrical phenomena have been studied since antiquity, though advances in the science were not made until the seventeenth and eighteenth centuries. Practical applications for electricity however remained few, and it would not be until the late nineteenth century that engineers were able to put it to industrial and residential use. The rapid expansion in electrical technology at this time transformed industry and society. Electricity's extraordinary versatility as a source of energy means it can be put to an almost limitless set of applications which include transport, heating, lighting, communications, and computation. Electrical power is the backbone of modern industrial society, and is expected to remain so for the foreseeable future.

Electric Current:

The movement of electric charge is known as an electric current, the intensity of which is usually measured in amperes. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current.

By historical convention, a positive current is defined as having the same direction of flow as any positive charge it contains, or to flow from the most positive part of a circuit to the most negative part. Current defined in this manner is called conventional current. The motion of negatively charged electrons around an electric circuit, one of the most familiar forms of current, is thus deemed positive in the oppositedirection to that of the electrons. However, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation.

The process by which electric current passes through a material is termed electrical conduction, and its nature varies with that of the charged particles and the material through which they are travelling. Examples of electric currents include metallic conduction, where electrons flow through a conductor such as metal, and electrolysis, where ions (charged atoms) flow through liquids. While the particles themselves can move quite slowly, sometimes with an average drift velocity only fractions of a millimetre per second, the electric field that drives them itself propagates at close to the speed of light, enabling electrical signals to pass rapidly along wires.

Current causes several observable effects, which historically were the means of recognising its presence. That water could be decomposed by the current from a voltaic pile was discovered by Nicholson and Carlisle in 1800, a process now known as electrolysis. Their work was greatly expanded upon by Michael Faraday in 1833. Current through a resistance causes localised heating, an effect James Prescott Joule studied mathematically in 1840. One of the most important discoveries relating to current was made accidentally by Hans Christian Ørsted in 1820, when, while preparing a lecture, he witnessed the current in a wire disturbing the needle of a magnetic compass. He had discovered electromagnetism, a fundamental interaction between electricity and magnetics.

In engineering or household applications, current is often described as being either direct current (DC) or alternating current (AC). These terms refer to how the current varies in time. Direct current, as produced by example from a battery and required by most electronic devices, is a unidirectional flow from the positive part of a circuit to the negative. If, as is most common, this flow is carried by electrons, they will be travelling in the opposite direction. Alternating current is any current that reverses direction repeatedly; almost always this takes the form of a sine wave. Alternating current thus pulses back and forth within a conductor without the charge moving any net distance over time. The time-averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the reverse. Alternating current is affected by electrical properties that are not observed under steady state direct current, such as inductance and capacitance. 

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Simple Harmonic Motion

Simple Harmonic Motion(SHM) is a type of oscillatory motion. It is where restoring force is directly proportional to the displacement. The motion of a simple pendulum as well as molecular vibration are examples of SHM. 

Simple Pendulum executing
SHM

A simple harmonic oscillator is attached to the spring, and the other end of the spring is connected to a rigid support such as a wall. If the system is left at rest at the equilibrium position then there is no net force acting on the mass. However, if the mass is displaced from the equilibrium position,a restoring elastic force which obeys Hooke's law is exerted by the spring.

Mathematically, the restoring force F is given by

where F is the restoring elastic force exerted by the spring (in SI units: N), k is the spring constant (N·m⁻¹), and x is the displacement from the equilibrium position (in m).

For any simple harmonic oscillator:

• When the system is displaced from its equilibrium position, a restoring force which resembles Hooke's law tends to restore the system to equilibrium.

Once the mass is displaced from its equilibrium position, it experiences a net restoring force. As a result, it accelerates and starts going back to the equilibrium position. When the mass moves closer to the equilibrium position, the restoring force decreases. At the equilibrium position, the net restoring force vanishes. However, at x = 0, the mass has momentum because of the impulse that the restoring force has imparted. Therefore, the mass continues past the equilibrium position, compressing the spring. A net restoring force then tends to slow it down, until its velocity vanishes, whereby it will attempt to reach equilibrium position again.

As long as the system has no energy loss, the mass will continue to oscillate. Thus, simple harmonic motion is a type of periodic motion.

Formulae:

From the above equation F = -k.x where k is the spring constant and the -ve sign is to show that the Force is towards the equilibrium.

F = -k.x

F = ma

==> m.a = -k.x

==> m.a + k.x = 0

==> a + (k/m).x = 0

but  k/m = w²

Therefore

where ω is the angular frequency

therefore ω = 2πf

Acceleration as a function of displacement:

and

Velocity is maximum at equilibrium position. Displacement and acceleration are maximum at the maximum amplitude though they are independent of amplitude. 

Mass on a spring

A mass m attached to a spring of spring constant k exhibits simple harmonic motion in space. The equation

shows that the period of oscillation is independent of both the amplitude and gravitational acceleration

Uniform circular motion

Simple harmonic motion can in some cases be considered to be the one-dimensional projection of uniform circular motion. If an object moves with angular velocity ω around a circle of radius r centered at the origin of the x-y plane, then its motion along each coordinate is simple harmonic motion with amplitude r and angular frequency ω.

Mass on a simple pendulum

In the small-angle approximation, the motion of a simple pendulum is approximated by simple harmonic motion. The period of a mass attached to a string of length ℓ with gravitational acceleration g is given by

This shows that the period of oscillation is independent of the amplitude and mass of the pendulum but not the acceleration due to gravity (g), therefore a pendulum of the same length on the Moon would swing more slowly due to the Moon's lower gravitational acceleration.

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Oscillations

When a simple pendulum is pushed through a small distance and observed, we find that the pendulum keeps moving to and fro about its initial position. It moves through its initial position, moves the farthest from there to a maximum height and stops for a few milliseconds(observable) and moves again through its initial position to the other end of the same height(without friction). Thus we can describe the pendulum to be oscillating about it mean position.

Oscillation is a periodic to and fro motion of an object about its mean position. Oscillation is also called as vibration. Another example is shown below.

Spring mass oscillatory
system

The simplest mechanical oscillating system is a mass attached to a linear spring subject to no other forces. Such a system may be approximated on an air table or ice surface. The system is in an equilibrium state when the spring is static. If the system is displaced from the equilibrium, there is a net restoring force on the mass, tending to bring it back to equilibrium. However, in moving the mass back to the equilibrium position, it has acquired momentum which keeps it moving beyond that position, establishing a new restoring force in the opposite sense. If a constant force such as gravity is added to the system, the point of equilibrium is shifted. The time taken for an oscillation to occur is often referred to as the oscillatory period or the time period.

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Uses of gravity

Gravitational force is needed a lot. We depend on gravity for many things.

1. We are able to stand on the Earth's surface only because of gravity. The Gravitational force keeps us bound to the Earth's surface and does not let us escape to vacuum though rockets have enough power to do so. Gravity varies from place to place.
2. The fluids in our body are right in their places because gravity holds them.
3. The muscles in our body can be made stronger only because of gravity.
4. The Sun, Moon, Earth and celestial objects are in the orbits only because of gravity.
5. The satellites which are supposed to reach far away planets like Uranus, Jupiter etc use the centripetal force exerted by the gravity to gain momentum and travel farther distances.
6. The light from a distant planet is bent by the gravity of a celestial object which helps us to calculate the distance of the Star.
7. A stellar system with a planet can be found out using Doppler effect which works on Gravity.

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Gravitational Energy

Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, and coalesced matter to remain intact, thus accounting for the existence of the Earth, the Sun, and most of the macroscopic objects in the universe.

The motion of celestial bodies such as the moon, the Earth, the Planets, etc., has been a subject of great interest for a long time. Famous Indian Mathematician and Astronomer, Aryabhatta, studied these motions in great detail(5th century A.D). He established that the Earth revolves its own axis around the Sun and the Moon also revolves around the Earth. 

A thousand years later, Tycho Brahe and Johannes Kepler. Kepler formulated his important findings and put it as three laws of planetary motion.

1. All planets move in elliptical orbits around the sun at a focus.
2. The radius vector from the sun to the planet sweeps equal areas in equal time.
3. The square of the time period of a planet is proportional to the cube of the semi-major axis of the ellipse.

Newton tried to use these laws and formulated the law of gravitation. The formula is 

Gravitation is one of the four fundamental interactions of nature, along with electromagnetism, and the nuclear strong force and weak force. Modern physics describes gravitation using the general theory of relativity by Einstein, in which it is a consequence of the curvature of space-time governing the motion of inertial objects. The simpler Newton's law of universal gravitation provides an accurate approximation for most physical situations.

Equivalence Principle: The equivalence principle, explored by a succession of researchers including Galileo, Loránd Eötvös, and Einstein, expresses the idea that all objects fall in the same way. The simplest way to test the weak equivalence principle is to drop two objects of different masses or compositions in a vacuum, and see if they hit the ground at the same time. These experiments demonstrate that all objects fall at the same rate when friction (including air resistance) is negligible. More sophisticated tests use a torsion balance of a type invented by Eötvös. Satellite experiments, for example STEP, are planned for more accurate experiments in space.

Formulations of the equivalence principle include:

• The weak equivalence principle: The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition.
• The Einsteinian equivalence principle: The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in space-time.
• The strong equivalence principle requiring both of the above.

The equivalence principle can be used to make physical deductions about the gravitational constant, the geometrical nature of gravity, the possibility of a fifth force, and the validity of concepts such as general relativity and Brans-Dicke theory.

Gravity and Quantum Mechanics: In the decades after the discovery of general relativity it was realized that general relativity is incompatible with quantum mechanics. It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, such that the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required. Many believe the complete theory to be string theory, or more currently M-theory, and, on the other hand, it may be a background independent theory such as loop quantum gravity or causal dynamical triangulation.

Gravity in accordance with astronomy: The discovery and application of Newton's law of gravity accounts for the detailed information we have about the planets in our solar system, the mass of the Sun, the distance to stars, quasars and even the theory of dark matter. Although we have not traveled to all the planets nor to the Sun, we know their masses. These masses are obtained by applying the laws of gravity to the measured characteristics of the orbit. In space an object maintains its orbit because of the force of gravity acting upon it. Planets orbit stars, stars orbit Galactic Centers, galaxies orbit a center of mass in clusters, and clusters orbit in super-clusters. The force of gravity exerted on one object by another is directly proportional to the product of those objects' masses and inversely proportional to the square of the distance between them.

Gravitational Radiation: In general relativity, gravitational radiation is generated in situations where the curvature of spacetime is oscillating, such as is the case with co-orbiting objects. The gravitational radiation emitted by the Solar System is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as PSR B1913+16. It is believed that neutron star mergers and black hole formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as LIGO have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.

A comet in motion gradually will come to rest after billions of years though there is no friction in vacuum because it loses energy in the form of gravitational waves.

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Friction

Friction is a resisting force which opposes the movement or relative motion of two surfaces in contact with each other. Friction does not let two bodies slide easily.
There are several types of friction

1. Dry Friction: Friction that exists between two solid surfaces in contact.
2. Fluid friction or viscosity: Friction that exists between layers of a viscous fluid.
3. Skin friction: A force which opposes the movement of a solid through a fluid.
4. Internal Friction: Friction that exists in a solid which opposes its deformation.

The Force of Friction exists when a body is stationary as well as moving. The friction that exist when a body is stationary is called static friction. 

Static Friction makes a body stay in its position even though a small amount of external force is being applied(depends on the roughness of the surface). It is denoted by μ_s.

The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: . When there is no sliding occurring, the friction force can have any value from zero up to . Any force smaller than  attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than  overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable—the friction between the two surfaces is then called kinetic friction.

Friction between a body and a surface when the body is moving is called Kinetic friction. It is denoted by μ_k.

The Kinetic Friction is smaller than Static Friction.

Angle of Friction: For certain applications it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called the angle of friction or friction angle. It is defined as:

where θ is the angle from vertical and µ is the static coefficient of friction between the objects. This formula can also be used to calculate µ from empirical measurements of the friction angle.

Static Friction as limiting friction:

Static Friction behaves rather differently in different situations. Experiments show that static friction increases with the increase in external forces being applied. The friction keeps increasing until it meets a certain value or reaches its limit. Then the friction is no longer static. Its kinetic as told above. Thus we can conclude that its is harder to begin the motion of an object rather than keeping it in motion.

Calculating Force of Friction: 

Friction between each surfaces can be calculated using an equation or idea given by Charles-Augustin de Coulomb. The equation is .

where

•  is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction opposite to the net applied force.
•  is the coefficient of friction, which is an empirical property of the contacting materials,
•  is the normal force exerted by each surface on the other, directed perpendicular (normal) to the surface.

Concept of Normal Force: The normal force is defined as the net force compressing two parallel surfaces together; and its direction is perpendicular to the surfaces. From Newton's third law of motion, we can say that Normal force is a reaction to the force an object is applying on the surface.

In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where . In this case, the magnitude of the friction force is the product of the mass of the object, the acceleration due to gravity, and the coefficient of friction. However, the coefficient of friction is not a function of mass or volume; it depends only on the material. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block. However, the magnitude of the friction force itself depends on the normal force, and hence the mass of the block.

If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force  between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g. If the object is on a tilted surface such as an inclined plane, the normal force is less, because less of the force of gravity is perpendicular to the face of the plane. Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram. Depending on the situation, the calculation of the normal force may include forces other than gravity.

Laws of Friction:

The elementary properties of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuries and were expressed as three empirical laws:

• Amontons' First Law: The force of friction is directly proportional to the applied load.
• Amontons' Second Law: The force of friction is independent of the apparent area of contact.
• Coulomb's Law of Friction: Kinetic friction is independent of the sliding velocity.

Amontons' 2nd Law is an idealization assuming perfectly rigid and inelastic materials. For example, wider tires on cars provide more traction than narrow tires for a given vehicle mass because of surface deformation of the tire.

Fluid Friction: Fluid friction occurs between layers within a fluid that are moving relative to each other. This internal resistance to flow is described by viscosity. In everyday terms viscosity is "thickness". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement.

All real fluids (except superfluids) have some resistance to stress and therefore are viscous, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.

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