Professor G. S. Mehmi

Terminal velocity : According to Stoke's law when a spherical body falls through a viscous liquid, it experiences a viscous force and under the combined effect of viscous force, weight of body and upthrust on the body by the liquid, the body moves with a constant velocity in the liquid is called terminal velocity. Relation of terminal velocity is v  =  2.r.r.( ρ - σ ).g/9 η

Stoke's law : According to this law, when a spherical body of radius (r) moves with velocity (v) in a liquid of coefficient of viscosity ( η ). then the viscous force acting on the body is given by relation  F = 6 πηrv  This relation is called Stoke's law and gives the magnitude of the viscous force acting on a body.

Viscosity : It is the property of liquids (or gases) by virtue of which an internal resistance or friction comes into play when a liquid is in motion. The internal friction tries to oppose the motion of the liquid and liquid comes to rest after sometime. Relation              F =  - η. A. (dv)/(dx) Where η is known as Coefficient of Viscosity  and negative sign shows that the direction of viscous force is opposite to the direction of motion of liquid. Coefficient of Viscosity ( η) of a liquid may be defined as the tangential viscous force which maintains a unit velocity gradient between two parallel layers of a liquid each of unit area. Units of Coefficient of Viscosity ( η) : In C.G.S. system,  the unit of co-efficient of viscosity is poise. In S.I. system,  the unit of co-efficient viscosity is decapoise.

Pressure: It is defined as the magnitude of force acting normally per unit area of the surface of body. SI unit of pressure is N/m² and is called Pascal (Pa). Atmospheric pressure : The atmospheric pressure at any point is numerically equal to the weight of a column of air of unit cross-sectional area extending from that point to the top of atmosphere. Gauge pressure and Vacuum pressure : When the unknown pressure is higher than the atmospheric pressure, the pressure recorded by the instrument is called gauge pressure while a pressure recorded below the atmospheric pressure is known as vacuum pressure.

Elasticity = Stress / Strain Elasticity ∝  1 / Strain This means elasticity will be more, if strain produced is less that is, for a smaller change in shape or size under the action of a large force corresponds to higher value of elasticity. Let us have equal volume of air and water, and we apply same pressure on both, strain produced in water will be less, than strain produced in air. As a result, the elasticity of water is more than elasticity of air.

A better spring must have large restoring force when deformed. Restoring force depen upon the elasticity of the material of the spring. As the Young's modulus of steel is greae than that of copper, so large restoring force is developed in steel in comparison to copper when external applied force (Deforming force) is removed. This fulfils the purpose for which a spring is required. So this is the reason why steel is preferred to copper in making a spring. Note : Steel is also more elastic than rubber.

Strain is defined as the ratio of change in dimensions of the body to the original dimensions of body. Strain = Change in dimensions (Length or Area or Volume) / Original dimensions (Length or Area or Volume) It has no units and dimensions. Types of strains: Strain has three types : (1) Longitudinal strain (2) Volumetric strain (3) Shear strain. (1) Longitudinal strain: It is defined as the increase or decrease in length per unit original length of body when deformed by an external force. This strain is also called linear strain. If l is change (increase or decrease) in length of body on applying deforming force and L is original length of body, then the longitudinal strain is given by, Longitudinal strain = Change in length (l)/ Original length (L) It has no units and dimensions. (2) Volumetric strain: It is defined as the change (increase or decrease) in volume per original volume of body when deformed by an external force. If ΔV is change (increase or decrease) in volume of...

Stress: It is defined as the restoring force acting per unit area of the body deformed by external force. Stress = Restoring force/ Area = F/A where F is restoring force and A is area of body. Types of Stresses: Two types 1. Normal stress: The restoring force (or deforming force) acting per unit area normal to the surface of body is called the normal stress. For e.g. When a wire is pulled by a fores along the length of wire, the stress is normal stress. Normal stress is of two types: (।) Longitudinal tensile stress: If length of the body increases in the direction of applied force, then stress is called longitudinal tensile stress. (ii) Compressional stress: If length of the body or volume of body decreases on applying deforming force, then restoring force per unit area is called compressional stress. 2. Tangential stress: The restoring force (or deforming force) acting per unit area tangential to the surface of body is called tangential stress. Units of stress:  CGS unit ...

According to this law , the extension produced in a loaded spring or wire is directly proportional to the load attached to it within elastic limit.          Extension  ∝  Load In terms of stress and strain, Hook's law can also be stated as,         Stress  ∝   Strain   Or  Stress =  constant × Strain   Or  Stress/ Strain = constant  =  Modulus of elasticity (E) The ratio of stress and strain for a body is always constant and is called modulus of elasticity or coefficient of elasticity (E). The value of this constant (E) depends upon the nature of the material of the body.

The principle of conservation of energy states that , "Energy can neither be created nor be destroyed but may be converted from one form to another form."  It is the fundamental law of Physics.  For e.g. 1. When a speeding car hits a wall, the car comes to rest. Here, the kinetic energy of car is not destroyed but is converted into heat energy, sound energy. It will be seen that kinetic energy of car before hitting is equal to produced heat or sound energy after hitting. This shows that total energy is the same though it has changed its form. 2. When a bullet hits a target, the kinetic energy of bullet is converted into sound energy. kinetic energy of bullet is exactly equal to sound energy produced after hitting.

  ਮੇਰੇ ਮਨ ਵਿੱਚੋਂ ਇਕ ਕਹਾਣੀ ਲੰਘੀ । ਇਕ ਟਿੱਡਾ ਪਰਵਾਨੇ ਨੂੰ ਆਖਣ ਲਗਿਆ ਮੈਂ ਸ਼ਮਾਂ ਨਾਲ ਸ਼ਾਦੀ ਕਰਨੀ ਚਾਹੁੰਦਾ ਹਾਂ । ਪਰਵਾਨੇ ਨੇ ਦੱਸਿਆ ਕਮਲਾ ਨਾ ਬਣ ਸ਼ਮਾਂ ਨੂੰ ਉਹੀ ਗਲ ਲਾ ਸਕਦਾ ਹੈ, ਜਿਹੜਾ ਜਲ ਜਾਣ ਤੋਂ ਨਾ ਡਰੇ । ਟਿੱਡਾ ਬੋਲਿਆ ਮੈਂ ਸਮਾਂ ਨੂੰ ਪਰਵਾਨਿਆਂ ਨਾਲੋਂ ਵੀ ਵਧ ਮੁਹੱਬਤ ਕਰਦਾ ਹਾਂ । ਪਰਵਾਨਾ ਬੋਲਿਆ ਚਲੋ ਮੰਨ ਲੈਂਦੇ ਹਾਂ । ਤੇਰੀ ਸਫ਼ਾਰਸ਼ ਸ਼ਮ੍ਹਾਂ ਕੋਲ ਕਰ ਦਿਆਂਗਾ । ਜਾ ਕੇ ਦੇਖ ਕੇ ਆ ਕਿ ਮਹਿਲ ਅੰਦਰ ਸ਼ਮ੍ਹਾਂ ਜਗ ਪਈ ਹੈ। ਟਿੱਡਾ ਜਾ ਕੇ ਦੇਖ ਆਇਆ ਤੇ ਆ ਕੇ ਬੋਲਿਆ, ਜਗ ਪਈ ਹੈ ਪਰਵਾਨੇ ਨੇ ਕਿਹਾ ਜੇ , ਜਗ ਪਈ ਸੀ ਤੂੰ ਮੁੜ ਕਿਸ ਲਈ ਆਇਆ ਹੈਂ ? ਮੈਂ ਉਸ ਨੂੰ ਕਹਿਣਾ ਚਾਹੁੰ ਦਾ ਸੀ ਤੂੰ ਹੱਥ ਸੇਕਣ ਲਈ ਅੱਗ ਮੰਗਦਾ ਏਂ, ਅੱਗ ਦਾ ਹੱਥ ਫੜਨ ਜੋਗਾ ਤੂੰ ਨਹੀਂ। ਪਰ ਕੁਝ ਨਾ ਕਿਹਾ ।

Friction is a necessary evil : In most of the cases friction is found to be of disadvantage. Due to friction the energy given to a machine is not totally converted into useful work but a part of it is converted into heat. Friction causes wear and tear of the moving parts of the machines and therefore is an evil. In many cases friction plays a very important role in our daily life. We are able to walk due to friction otherwise we will slip. We can not stop our cycle if there is no friction between the brakes and the rims. Thus friction is a necessity. Therefore, sometimes friction is an evil and at some stages it is very necessary in daily life. Thus, friction is necessary as well as an evil.