What is Force?
“A force moves or tends to move,stops or tends to stop the motion of object.The force can also change the direction of motion of an object.”It can also change the shape or size of a body on which it acts.
In classical physics real forces are defined by a set of axioms,Newton’s laws of motion,with reference to an inertial reference frame.By Newton’s second law the resultant force F acting on a body of constant mass m is equal to ma ,where a is the acceleration of a.Force is a vector quantity.
Forces are either long range or short range.Long range forces,such as gravitation and the coulomb force,fall off less rapidly than the inversion fourth power of the distance,short range forces,such as those inside the atomic nucleus and those between molecules,fall off more rapidly than the inverse fourth power.It can be shown that in a condensed body,short range forces are small at distances not much greater than those of near neighbors.
Some workers find it convenient to use fictitious forces in analyses.There are two kindsinertia forces and inertial forces.
An inertia force is a fictitious force that is supposed to act on a body,being equal and opposite to the resultant of the real forces.Since it does not represent any actual interaction,an inertia force does not obey newton’s third law.According to the principle of d’Alembert (1742) any accelerated body can be treated as if it were in equilibrium under the action of the real forces and the fictitious one.
When a problem is considered from the point of view of an observer who is accelerated with trespect to an inertial reference frame,Newtons’s laws are not applicable to real interactions.It is possible to apply these laws in such a case by introducing a fictitious force.In particular ,an observer on a rotating body may use an inertial force called a Coriolis force,which is supposed to act at right angles to the path of a body that moves towards or away from the axis of rotation.Problems on projectiles and movements of the atmosphere and oceans are often treated in this way.
When a particle of mass m moves in a circular arc of radius r with uniform angular velocity ω,there is an acceleration rω² towards the centre,so by Newton’s second law the resultant force on the particle,called the centripetal force,is mrω²,acting radially inward.In the system of Alembert there is a fictitious inertia force on the equal to this,supposelly acting radially outwards.Also an observer orbiting with the particle could introduce an inertial force that would be ,in this case,equal and opposite to the centripetal force.Both fictitious forces may be called centrifugal forces since they are directed away from the centre .
Now the centripetal force is real,so there must be an equal and opposite real force acting on another body,by newton’s third law.This real force is also often called a centrifugal force.
In kinematics,we have studied the changes in motion only.Our understanding about the changes in motion is of little value without knowings its causes.The branch of mechanics that deals with the study of motion of an object and the cause of its motion is called dynamics.In this section we shall study momentum and investigate what causes a change in the motion of a body and what role the mass of body plays in its motion.This inquiry leads us to the concept of force.We shall also study the newton’s laws of motion and their applications.Newton’s laws of motion are of fundamental importance in understanding the causes of motion of a body.Before we discus newton’s laws,it is appropriate to understand force,inertia and momentum.
We can open a door either by pushing or pulling it.The push may move the cart or change the direction of its motion or may stop the moving cart.A batsman can change the direction of a moving ball by pushing it with his bat.A force may not always cause a body to move.It can also stops the body,it can also change the direction of body.
Units of force:
The SI unit of force is newton denoted by N.According to newton’s second law of motion:”One newton (1 N) is the force that produces an acceleration of one meter per second square in a body of mass 1 kg.Thus,a force of one newton can be expressed as:
1N = 1kg 1ms^{2}
1N = Ikgms^{2}
Two other systems of units in common use are the cgs (centimetergramsecond) and the british systems.In the cgs system the unit of force is dyne and is equivalent to the g cms^{2}.Since 1kg=1000 g and 1ms^{2}=100 cms^{2},it follows that 1N =105 dyne.A dyne is a very small unit,roughly equal to the weight of a cubic millimeter of water.(A newton on the other hand,is about the weight of a half cup of water).
In british system,force is measured in pounds and acceleration in ft/s^{2}.In this system,the mass that is accelerated at 1ft/s^{2 }by a force of 1 lb is called the slug (from the word sluggish^{,}meaning slow or unresponsive).
Table of unit of force in different systems of unit.
System  Unit of force 
SI  Newton (N) 
Cgs  Dyne 
British  Slug 
Other variants on these basic systems are occasionally found,but these three are by far the most common.
Measuring forces:
Force is measured in dynamic method in which acceleration imparted to a standard body by pulling on it with a stretched spring.Although convenient for the purposes of definition,it is not always a particularly practical procedure for the measurement of forces.(Acceleration is seldom easy to measure.) Another method for measuring forces is base on measuring the change in shape or size of a body (a spring,say) on which the force is applied when the body is un accelerated .This may be called the static method of measuring forces.
The basis of the static method is that when a body under the action of several forces,has zero acceleration,the vector sum of all the forces acting on the body must be zero.This is, of course,just the second law of motion.A single force acting on a body would produce an acceleration;this acceleration can be made zero if we apply another force to the body equal in magnitude but oppositely directed.In practice we seek to keep the body at rest.If now we choose some force as our unit force,we are in a position to measure forces.The pull of the earth on a standard body at a particular point can be taken as the unit force.
Types of force with examples:
Basically forces are of two types:
 Contact forces
 Non contact forces or At a distance forces

Contact forces:
Those forces which represent the result of physical contact between two objects,where one of the object exerts force on the other.Stretching of the spring of a balance,pushing of a pram and kicking of a football etc are some examples of contact forces.Other some types of contact forces are given in the list below:
 Applied force
 Normal force
 Frictional force
 Tension force
 Air resistance force
 Spring force

Applied force:
Force due to the action of muscles is called applied force.It is also called muscular force.
Normal force:
It is a contact interaction force between surfaces.It always acts perpendicular to the surfaces and out of the surface.It comes from the microscopic deformation of molecules modeling a system of springs.
Frictional force:
Frictional force comes from interactions with a surface as an object moves or tries to move relative to the surface.
Tension force:
An applied force where the force is applied through a string,cable,rope etc..
Note that a tension force can only pull,it cannot push.We usually assume the tension in a cable is the same everywhere in the cable.
Air resistance force:
Force that acts in a direction opposite motion through a gas is called air resistance force.It comes from cumulative interaction with air molecules.It increases as the velocity through the gas increases.It also increases as the area normal to the direction of motion increases.
Spring force:
Spring force comes from displacement of molecules.It is always opposite the displacement of spring.
Non contact forces:
Those forces which do not involve physical contact between the two objects but act through the space between the two.Examples of non contact or action at a distance forces,which are given in the list given below:
 Gravitational force
 Electromagnetic force
 Weak nuclear force
 Strong nuclear force
Gravitational force of attraction:
Attractive force exerting between two objects is called gravitational force.It originates with the presence of matter.
Electromagnetic force:
The electromagnetic force which includes basic electric and magnetic interactions and is responsible for the binding of atoms and the structure of solids.
Weak nuclear force:
The weak nuclear force which cause certain radioactive decay processes and certain reactions among the most fundamental particles.
Strong nuclear force:
The strong force which operates among the fundamental particles and is responsible for bonding the nucleus together.
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