Professor G. S. Mehmi

Force of friction between two surfaces obey the following laws : 1. Friction is an opposing force and comes into play, only if the body is in motion or tending to move over the surface of another body. 2. Force of friction acts along the tangent to the two surfaces at the point of contact. 3. The force of friction acts in a direction opposite to the direction of motion of the body. 4. The force of friction is independent of the areas of the surfaces in contact. 5. The force of friction is directly proportional to the normal reaction between the two surfaces in contact i.e. f ∝ R. Co-efficient of Friction : The ratio of the limiting (maximum) force of friction to the normal reaction is called co-efficient of friction. We know  f ∝ R               or f = μ R where μ is constant of proportionality and is called the co-efficient of friction. And  μ = f/R = (Force of limiting friction)/(NormalReaction)....

FRICTION : The opposing force which comes into play in between the surfaces of two bodies when one body is moved over the surface of another body is called force of friction or friction. Whenever there is a relative motion between the two surfaces in contact, an opposing force comes into play to prevent the relative motion. This opposing force is called force of friction. Cause of Friction: The surfaces of solid bodies are not perfectly smooth. Friction is produced by the interlocking of irregularities of one surface with that of the other surface in contact. Types of Friction : 1. Static Friction : The friction offered by the surfaces in contact subjected to external forces until there is no motion between them is called static friction. 2. Limiting Friction : The maximum value of friction which comes into play when a body just starts moving over the surface of another body is called limiting friction. 3. Dynamic Friction : The force acting between the surfaces when there is a ...

ਨਿੱਤ ਸੂਰਜ ਆ ਕੇ ਪੁੱਛਦਾ ਅੱਜ ਦਾ ਦਿਨ ਕਿਸ ਅਰਥ ਲਾਉਣਾ ? ਨਿੱਤ ਤਾਰਿਆਂ ਨੂੰ ਮੈਂ ਦਸਦਾ ਅੱਜ ਦਾ ਦਿਨ ਬੇਅਰਥ ਗੰਵਾਇਆ : ਅਰਥ ਤੋਂ ਬੇਅਰਥ ਤੱਕ ਦਾ ਪੈਂਡਾ ਨਿੱਤ ਹੰਢਾਵਾਂ ਹਰ ਰਾਹ ਇਕ ਵਿਸ਼ਵਾਸ ਤੇ ਹਰ ਵਿਸ਼ਵਾਸ ਇਕ ਧੋਖਾ। ਆਪਣੀ ਛਾਵੇਂ ਕੋਈ ਬੈਠ ਨਾ ਸਕਦਾ । ਕੁਝ ਅਰਥ ਜੋ ਕਦੇ ਸ਼ਬਦ ਨਾ ਬਣਦੇ ਕੁਝ ਸ਼ਬਦ ਨੇ ਜੋ ਅਰਥਾਂ ਤੋਂ ਖ਼ਾਲੀ ਤੇਰੀ ਥਾਵੇਂ ਮੇਰੀ ਥਾਵੇਂ ਤੇਰੇ ਰਾਹੀਂ ਮੇਰੇ ਰਾਹੀਂ ਇਸ ਜੱਗ ਉਤੇ ਜੀਉਂਦੇ ਤੂੰ ਮੈਂ ਬਿਨ ਜੀਉਦਿਆਂ ਮਰ ਜਾਣਾ । ਆਪਣੀ ਛਾਵੇਂ ਕੋਈ ਬੈਠ ਨਾ ਸਕਦਾ । ਆਪਣੀ ਛਾਵੇਂ ਕੋਈ ਬੈਠ ਨਾ ਸਕਦਾ ।

Principle of Homogeneity of Dimensions : This principle states that, "In a dimensionally correct physical relation the dimensions of each and every term on both sides of the relation must be same". According to this principle ; (1) We can add or subtract only those physical quantities which have same nature (means if they have same dimensions). (2) A physical relation, must be dimensionally homogeneous i.e., all the terms on both sides of the relation must have the same dimensions.

  Systems of Fundamental Units : 1. CGS System. In this systems, the unit of length is centimetre, the unit of mass is gram and the unit of time is second. 1m = 100 cm  or  1cm = 1/100 m. 1kg = 1000g   or   1g = 1/1000 kg. 1second =1/86400th part of a day. 2. FPS System. In this system, the unit of length is foot, the unit of mass is a pound and unit of time is second. 1 foot  = 12 inch = 12 x 2.54 cm = 30.48 cm 1 pound = 453 g = 0.453 kg. 3. MKS System. In this system the unit of length is metre, the unit of mass is kilogram and the unit of time is second. 4. SI Units. French name of this system is System International de Units  while in English SI stands for International System of Units. This system is an improved and extended version of MKS system of units. This system is based on seven base and two supplementary units. Base Units: 1. Length  -  metre (m). 2. Mass  -  kilogram (kg). 3. Time   -...

Unit :  The chosen standard of the same kind (as that of given physical quantity) taken as reference in the measurement of a physical quantity is called unit. Characteristics of a standard unit : (i) It should be well defined. (ii) It should be of suitable size. (iii) It should be easily reproducible at all places. (iv) It should not change with time and from place to place. (v) It should be easily accessible. Types of unit : Two types (i) Fundamental Units (ii) Derived Units. (i) Fundamental Units : The units selected for measuring the fundamental quantities (mass, length and time) are called fundamental units. Unit of mass is kilogram(kg). Unit of length is metre(m). Unit of time is second(s). (ii) Derived Units : T he units of derived physical quantities(area, speed,velocity, force etc.)are called derived units because they have derived from fundamental units. For e.g. Speed = Distance/Time  = m/s Therefore  m/s  is the Derived Units.

Centripital force : The force acting along the radius towards the centre of a circle to keep a body moving with uniform speed in a circular path is called centripetal force. Or The force which acts along the radius, towards the centre of a circular path and keep the body maving in circular path called centriped force. Centrifugal force : The force which acts along the radius but away from the centre of the circular path and keeps the body moving in a circular path is called centrifugal force. Application of Centrifugal force : * Cream separator : It is a device used to separate the lighter contents of milk i.e., cream from milk. When milk is rotated in a cylinder at very large speed, the heavier particles move toward the wall of the cylinder where as the lighter particles i.e., cream collects near the axis of the cylinder from where it can be removed very easily. * Drying machine : With the help of such machines the clothes can be dried very quickly. The clothes are placed in a...

Mass : Mass is the quantity of matter possessed by a body. * Mass of a body is a fundamental quantity.  * Mass is a scalar quantity.  * If a body exists in this universe, then its mass can never be zero. Weight : Weight of a body is the force with which a body is gravitationally attracted towards earth. Mathematically  relation  of Weight is     W= mg * Weight of a body  is a derived quantity.  * Weight is a vector quantity.  * Weight of a body can be zero. e.g. when g = 0 then W = 0.

1. Triangle Law of Vectors : If two vectors can be represented both in magnitude and direction by the sides of a taken in the same order, then their resultant is represented completely in magnitude and direction, by the third side of the triangle taken in the opposite order. Shown in fig. Resultant vector(R) is equal to sum of two vectors(P,Q). Relation is Case : If three vectors are represented by all three sides of a triangle taken in same order then their resultant is zero. Shown in fig. And that has relation is  2. Parallelogram Law of Forces : If two forces acting simultaneously on a body can be represented completely (in magnitude and direction) by the two adjacent sides of a parallelogram drawn from a point then their resultant is represented completely (in magnitude and direction) by the diagonal of the parallelogram passing through the same point of intersection. Shown in fig. The magnitude of resultant force is Direction of resultant force is 3. Polygon Law of Forces : ...

Scalar quantities: Those physical quantities which have magnitude but no direction are called scalar quantities. e.g. Area, volume, mass, speed, density, distance, charge, energy, temperature, length, time, etc. * Scalar quantities can be added, subtracted or multiplied by using simple rules of algebra. * Scalar quantities change if their magnitude is changed.  * Scalar quantities are represented by ordinary letters. Vector quantities: Those physical quantities which have magnitude as well as direction are called vector quantities. e.g. Displacement, velocity, acceleration, momentum, force, impulse, electric field, gravitational field etc. * Vectors do not follow the laws of algebra, i.e., vectors can not be added, subtracted or multiplied by the way of scalars. Vectors can be added, subtracted or multiplied by using rules of vector algebra. The division of a vector by another vector is not a valid operation in vector algebra. * Vector quantities change if either their magnitude or...