Physics Measurement Units, Quantities and Definitions

 Physics: It is the study of properties of Matter and Energy. It traditionally covers the subjects of mechanics, electricity and magnetism, heat, light and sound and hence called classical physics. Relativistic mechanics and Quantum theory have given rise to Modern Physics. Main subjects in Modern Physics are Atomic, Nuclear and Particle Physics.

Measurement: It defines any specific physical quantity in terms of other well-defined quantities, which can be measured accurately.

Unit: It is a conventionally fixed value or magnitude of a physical quantity in terms of which that quantity is expressed in measurements.

Parental or Fundamental Units: Three fundamental Units are length (L), mass (M), and time (T).

Child or Derived Units:  These are units which derived from fundamental units. These are velocity, acceleration, force, volume, area, etc.

Systems of Measurements: There are three systems of measurements. First CGS system (Centimeter, Gram, Second), Second FPS system (Foot, Pound, Second) and third MKS system (Meter, Kilogram, Second). MKS system is used for all practices.

SI Units: The International System of Units consists of six basic units. These are meter (length), kilogram (mass), second (time), ampere (current), Kelvin (temperature) and Candela (luminous intensity). All these are represented by m, kg, s, A, K and Cd respectively.

Standard Units: It is a unit which is established by choosing arbitrary values for some physical quantities and ten deriving units form them. For e.g. a certain quantity of matter can be selected as unit of mass.

•   Standard Meter: It is equal to 1,650,763.73 wavelengths of electromagnetic radiation emitted by the isotope ⁸⁶Kr in the transition between the states 2p₁₀ and 5d_5. It refers to particular physical states of krypton atom. Emitted Radiation can easily be identified because it appears as a red line in the spectrum. Thus this system does not change with time and place.

•   Standard Kilogram: The standard kilogram is the mass of a platinum-iridium cylinder stored in a special vault in Paris. For all practical purposes, it is equal to the mass of 10^-3 m³of distilled water at 4⁰C.

•  Standard Second: According to International Astronomical Union second is defined as 1/31,556,925.975 of the duration of the tropical year. It is also defined as 1/86,400^th part of the mean solar day. Second is also defined as the time required for the oscillator which forces cesium atom to oscillate 9,192,631,770 times.

 

Motion: When an object changes its position with respect to a fixed point within a certain time limit, the object is said to be in motion. Motion can be 2 types: linear and rotational (spin). Car on the road is the example of linear motion whereas motion of top, spinning on its axis, is an example of rotational motion.

Scalar Quantities: The quantities which have only magnitude are called scalar quantities, e.g., speed, mass, area, work, and energy.

Vector Quantities: The quantities which have both magnitude and direction are called vector quantities, e.g. velocity, acceleration, and force.

Displacement: The distance moved by an object in a particular direction is called displacement.

Speed: It is the rate of change of displacement, i.e. the distance it covers per unit of time.  Speed=Distance travelled/Time required. Its unit is m/sec.

Velocity: It is the rate of change of displacement along a particular direction. It is a vector quantity. Its unit is m/sec.

Relative Velocity: The relative velocity of a body with regard to anybody is the rate of change of position of that body with reference to the other, when both are in motion. It is obtained by compounding the absolute velocity of first body with the reversed velocity of the second body.

Acceleration: The rate of change of velocity with respect to time is called acceleration. Its unit is m/sec² and it is a vector quantity. Acceleration = Change in velocity/Time. Usually the term acceleration refers to increase in velocity of a body, while decrease in velocity is referred as retardation or deceleration.

Acceleration Due to Gravity: When an object is dropped from a height, it does not fall with a uniform velocity, but with an increased velocity. It happens because of acceleration due to gravity. The value of acceleration due to gravity (g), on the surface of the earth, is about 9.8 m/sec². It implies that when an object falls freely, its velocity increases every second by 9.8 m/sec. The value of g varies from place to place. The value of g is maximum at poles of the earth and minimum at the equator. At the center of the earth, g would become zero.

Relations:  If u is the initial velocity, v is the final velocity during the time‘t’ with constant acceleration ‘a’ and the distance traveled is ‘s’, then

1. v = u + at
2. s = ut + ½ at²
3. v²- u ²= 2as

  

 

Laws of Motion

First Law or Law of Inertia: - Each body continues to move in its state of rest or of continued motion in a straight line, except in so far as it is compelled by the impressed force to change that direction.

Second Law: - The Rate of change of momentum is proportional to impressed force and takes place in the direction of the force.

Third Law: - To each action there is an equal and opposite reaction.

Applications of Newton’s Laws

• If a cantering horse suddenly stops, the rider, if he is not firmly seated, is thrown forward (Inertia of Motion). 

•  An athlete runs for some distance before taking a jump so that his forward momentum may help him to achieve a longer jump (Inertia of Motion).

• Dust is removed from a hanging carpet by beating it with a stick because the dust particles tend to remain at rest and fall off as the carpet suddenly moves forward (Inertia of Rest)

•  When a man jumps out of a running bus, he falls forward on the road. Reason: While in the bus, the man shares the motion of the bus. As he jumps out, his lower portion comes to rest on touching the ground but the upper portion keeps on moving in the same direction of bus. Hence he falls, forward. In order to save himself falling down, the passenger runs for some distance on the road in the same direction (Inertia of Motion)

• When a bus at rest suddenly starts, a passenger falls or leans backward. Reason: With the bus at rest, passenger’s whole body was also at rest. As the bus starts moving, the lower part of passenger’s body, which is in contact with the bus, begins to move with the bus while the upper part remains at rest and is hence thrown backward (Inertia of Rest)

• When a men jumps from a boat, the boat moves back due to reaction as man jumps from a boat to ground (Third Law)

• When a shot is fired from the gun, the gun recoils backward due to reaction of shot on the gun. Many police recruits thus hurt their collar bone during initial firing practice with .303 rifles (Third Law)

•  While walking on the ground, we press backward on the ground with our feet, and the reaction on the ground gives us an equal and opposite force which makes us move (Action and reaction – Third Law)

•    If we try to open a door by kicking it, the foot gets hurt. Reason: The force exerted by us on door accelerates the door and it opens. But at the same time, the door exerts an equal and opposite force on us which reduces forward velocity of foot. The opposite reaction is responsible for the feeling of being hurt.

• While sitting on chair, we cannot lift the chair up by applying upward force on its arms. As we exert an upward force on chair, the chair exerts an equal and opposite reaction on us (Third Law).