physics

physics -
              the science of matter and energy and their interactions.


≡natural philosophy


↔phase space - (physics)
                                   an ideal space in which the coordinate dimensions represent the variables that are required to describe a system or substance; "a multidimensional phase space".


↔containment - (physics)
                                    a system designed to prevent the accidental release of radioactive material from a reactor.


↔hodoscope - (physics)
                                 scientific instrument that traces the path of a charged particle.


↔magnet - (physics)
                            a device that attracts iron and produces a magnetic field.


↔meniscus - (physics)

                                 the curved upper surface of a nonturbulent liquid in a vertical tube.


↔nuclear reactor, reactor - (physics)

                                                  any of several kinds of apparatus that maintain and control a nuclear reaction for the production of energy or artificial elements.


↔metastability -
                        the quality of a physical system that persists in its existing equilibrium when undisturbed (or only slightly disturbed) but able to pass to a more stable equilibrium when sufficiently disturbed.


↔isotropy, symmetry - (physics)
                                              the property of being isotropic; having the same value when measured in different directions.


↔wave-particle duality, duality - (physics)

                                                             the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory.


↔absorption factor, absorptivity - (physics)

                                                             the property of a body that determines the fraction of the incident radiation or sound flux absorbed or absorbable by the body.


↔reluctivity - (physics)              

                                the resistance of a material to the establishment of a magnetic field in it.


↔rest mass - (physics)
                               the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body.


↔relativistic mass - (physics)
                                        the mass of a body in motion relative to the observer: it is equal to the rest mass multiplied by a factor that is greater than 1 and that increases as the magnitude of the velocity increases.


↔gravitational mass - (physics)
                                             the mass of a body as measured by its gravitational attraction for other bodies.


↔inertial mass - (physics)

                                    the mass of a body as determined by the second law of motion from the acceleration of the body when it is subjected to a force that is not due to gravity.


↔mass energy - (physics)
                                    the mass of a body regarded relativistically as energy.


↔flux density, flux - (physics)
                                             the number of changes in energy flow across a given surface per unit area.


↔absorbance, optical density, photographic density, transmission density - (physics)
                 a measure of the extent to which a substance transmits light or other electromagnetic radiation.


↔quantum - (physics)

                              the smallest discrete quantity of some physical property that a system can possess (according to quantum theory).


↔attracter, attractor - (physics)
                                            a point in the ideal multidimensional phase space that is used to describe a system toward which the system tends to evolve regardless of the starting conditions of the system.


↔Bose-Einstein statistics - (physics)
                                                   statistical law obeyed by a system of particles whose wave function is not changed when two particles are interchanged (the Pauli exclusion principle does not apply).


↔Dalton's law of partial pressures, law of partial pressures, Dalton's law - (chemistry and physics)

                                  law stating that the pressure exerted by a mixture of gases equals the sum of the partial pressures of the gases in the mixture; the pressure of a gas in a mixture equals the pressure it would exert if it occupied the same volume alone at the same temperature.


↔Boltzmann distribution law, Maxwell-Boltzmann distribution law - (physics)
                                                                                                          a 
law expressing the distribution of energy among the molecules of a gas in thermal equilibrium.


↔Fermi-Dirac statistics - (physics)
                                                 law obeyed by a systems of particles whose wave function changes when two particles are interchanged (the Pauli exclusion principle applies).


↔Charles's law, Gay-Lussac's law, law of volumes - (physics)
                                                                                     the density of an ideal gas at constant pressure varies inversely with the temperature.



↔Hooke's law - (physics)
                                   the principle that (within the elastic limit) the stress applied to a solid is proportional to the strain produced.


↔Kirchhoff's laws - (physics)
                                       two laws governing electric networks in which steady currents flow: the sum of all the currents at a point is zero and the sum of the voltage gains and drops around any closed circuit is zero.


↔law of gravitation, Newton's law of gravitation - (physics)
                                                                                  the law that states any two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.


↔law of thermodynamics - (physics)
                                                    a law governing the relations between states of energy in a closed system.
   ↔mass-energy equivalence - (physics)
                                                      the principle that a measured quantity of mass is equivalent (according to relativity theory) to a measured quantity of energy.


↔Planck's law - (physics)
                                   the basis of quantum theory; the energy of electromagnetic waves is contained in indivisible quanta that have to be radiated or absorbed as a whole; the magnitude is proportional to frequency where the constant of proportionality is given by Planck's constant.


2. physics - the physical properties, phenomena, and laws of something; "he studied the physics of radiation"



≡physical science

↔natural science -
                           the sciences involved in the study of the physical world and its phenomena.


↔acoustics -
                    the study of the physical properties of sound

Physics

Physics



Scalar
          A scalar is a quantity that has magnitude only.


Vector
           A vector is a quantity that has magnitude and direction.



The moment (or torque) of a force.
                                                      The turning effect of a force (or moment or torque) about a point is defined as the force x the perpendicular distance from the point to the line of action of the force,
i.e. moment = F × d.
UNIT: Nm.


The principle of moments.
                                       For a system to be in equilibrium,
Σ anticlockwise moments about a point = Σ clockwise moments about the same point.


Centre of gravity.
                              The centre of gravity is the single point within a body at which the entire weight of the body is considered to act.



Displacement
                      The displacement of a point B from a point A is the shortest distance from A to B, together with the  direction. 
UNIT: m.

Newton's Laws of Motion

Newton's Three Laws of Motion

Newton's First Law of Motion:

                                               I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Newton's Second Law of Motion:

                                               II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

                                        This is the most powerful of Newton's three Laws, because it allows quantitative calculations of dynamics: how do velocities change when forces are applied. Notice the fundamental difference between Newton's 2nd Law and the dynamics of Aristotle: according to Newton, a force causes only a change in velocity (an acceleration); it does not maintain the velocity as Aristotle held. 

                                               This is sometimes summarized by saying that under Newton,

                                       F = ma,

but under Aristotle

                                       F = mv, where v is the velocity.

                                                                               Thus, according to Aristotle there is only a velocity if there is a force, but according to Newton an object with a certain velocity maintains that velocity unless a force acts on it to cause an acceleration (that is, a change in the velocity). As we have noted earlier in conjunction with the discussion of Galileo, Aristotle's view seems to be more in accord with common sense, but that is because of a failure to appreciate the role played by frictional forces. Once account is taken of all forces acting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that are found to be in accord with the observations.

Newton's Third Law of Motion:

                                             III. For every action there is an equal and opposite reaction.

                                                This law is exemplified by what happens if we step off a boat onto the bank of a lake: as we move in the direction of the shore, the boat tends to move in the opposite direction (leaving us facedown in the water, if we aren't careful!).

Friction and Lubrication

                                                           Some times there is screeching sound while using a pulley or hand pump. Friction is a force. Whatever work we do, friction works opposite to our force. When we apply a force to move something, the friction opposes our force and there is no motion. When our force is greater than the friction force, the object starts moving. The parts where there is friction, they wear out fast. A smooth surface shows lots of uneven and rough surface under a microscope. When these rough parts rub on each other, it results in friction. Oiling and greasing reduces friction and wear & tear of parts. When there is dust or foreign particles in the area of contact, friction and wear & tear are high and rapid. To prevent this, we must first clean the friction area and put oil or grease there. 

Friction is not always bad. Sometimes we require friction.

For example: 

1) While walking friction helps us. If we put oil on the floor, we will fall down due to slippery floor. We need friction to be able to walk. 

2) It is commonly seen that in muddy area the wheel of a truck slips round and round because there is no friction between the tyres and ground. 

3) Flour mill works because of friction between belt and wheel. 

4) Bicycle can be stopped by putting on a brake, which works because there is friction. 

5) When we tie a knot in a rope, it is the friction which holds the knot. Thus friction is useful to some extent but it will reduce the efficiency of work. 

Factors affecting friction

The intensity of friction depends on following factors:

 i) The area involved in friction.

 ii) The pressure applied on the surfaces. 

     Force = Pressure ´

    Area Frictional force will increase, if the area of contact will increase or if pressure applied on the surface increased. 

Methods to reduce friction

i) Polish the contact surface. 

ii) Put oil or grease so that it fills in the small gaps of the flat parts. 

iii) Use ball bearings to reduce area of contact between rotating parts.

Lubrication

                


  Following methods can be used to reduce friction: 

                                                                       Oil is either thin or viscous. It depends upon SAE No. of oil. (SAE means Society of Automotive Engineers). If we use very viscous oil, it does not reach all the parts. Very thin oil will flows away easily and gets wasted. Grease is used in such cases. It is generally used around ball-bearing. Normal grease or oil is never used where there is high pressure, high temperature and high speed. Special lubricants are used in such cases. In cold season the oil becomes thick and in hot season it becomes thin. Therefore selection of lubrication also depends on the season. It is always advisable to refer operating manual of the equipment before selecting the lubricant.

 

 

Power

Power

 

                 Power is the rate of work done or rate of energy consumed. We can manually pedal a wheel of bicycle at 1 revolution per second. If we want to turn the same wheel with 10 revolutions per second, we need more power and will use engine. 

Power is the time rate of doing work (or expending energy).

P    =       W/t      

      =       Fd/t   

      =      mgh/t

The SI power unit is called watt (W) in honor of James Watt (1736-1819), a Scottish engineer who developed one of the first practical steam engines. A common unit of electrical power is the kilowatt (kW). Power tells you how fast work is being done or how fast energy is transferred.

The SI unit for power is watt. 
One watt is equal to 1 Newton-meter per second (Nm/s). 

If you were pushing on something with a force of 1 N, and it moved at a speed of 1 m/s, your power requirement would be 1 watt. 

British unit of power is horsepower.

The conversion is as follows: SI:

  Watts (W) 1000 W = 1 kW Kilowatt (kW) 
                      1kW = 1.341 hp Horsepower (hp)
                      1 hp = 0.746 kW

 

Work

 

Work is the application of a force over a distance. When we lift an object from the ground and put it on the shelf, we do work. The force is the weight of the object and distance is the height of the shelf. Similarly, when we push an object and moves it, we do work. The distance taken for calculating work has to be in the direction of force applied. 

Example: a weight of 50kg. is lifted to height of 5 meter. Calculate the work done. Work = force ´ distance through force is applied = (W kg ´ 9.8) ´ h (W = 50kg, h = 3m) = 50 ´ 9.8 ´ 3 = 1470 Nm
When we push an object and it moves through a distance L,
the work done is calculated as follows:
Work = force ´ distance through which force is applied 
W = F ´ L (unit of force is N, distance is in meter) 
W = F (N) ´ L (m) 
    = F ´ L (Nm) 

When we do work, we use energy. Work and Energy are closely related. Work is measured in the same unit of energy.

 

Torque

 

                    Torque is a force that tends to rotate or turn things. You generate a torque whenever, you apply a force to turn handle of diesel engine.

Torque = Force ´ Distance from the center. 

Unit of Torque = N ´ m = Nm. 

It is common experience, that a nut which is hard to move by hand can be easily rotated when a spanner of longer length is used. Increasing length of application of force from the center of shaft increases torque applied.

 

Mass and Weight

 Mass and Weight

                         Mass is defined as the measure of how much matter an object or body contains. It is measured in Gram (g). More the mass of object more is the gravitational force on the object. If we drop an object from a height, earth pulls it at the acceleration of 9.8m/s2

                            

                               Weight is the amount of force that earth exerts on us.

  

                                Acceleration is the rate of change of speed. This means speed of an object will increase by 9.8m every second. This means, if an object falls from a height to reach earth, after 10 second it would have achieve speed of 9.8 ×10 = 98 m/s. · Force causes acceleration, 

Sir Isaac Newton’s Second Law states that the acceleration (a) of an object is directly proportional to the force (F) applied, and inversely proportional to the object’s mass (m) Newton’s Second Law is usually summarized in equation form: 

 a = F/m, 

or F = ma 

Unit of force is derived as follows -

Unit of force F = m (Kg) × a (m/s2) 

                    = Kg m/s2 

                    = N 

To honor Newton’s achievement, the standard unit of force i.e kg m/s2 in the SI system is named as Newton (N). One Newton (N) of force is enough to accelerate 1 kilogram (kg) of mass at a rate of 1 meter per second square (m/s2). A kilogram is the amount of weight at which 1 N of force will accelerate at a rate of 1 m/s2. In practice, we measure weight, in terms of gms. or Kgs. But when weight is used as force, we must remember to measure it in terms of Newton.