What is force and how many types of force in nature ,(gravitation, electromagnetic, nuclear, weak..)

WHAT IS FORCE

 INTRODUCTION

Force is a word which we have all heard about. When you push or pull some object you exert a force on it. If you push a body you exert a force away from yourself; when you pull, you exert a force toward yourself. When you hold a heavy block in your hand you exert a large force; when you hold a light block, you exert a small force.

can nonliving bodies exert a force ? Yes, they can. If we stand in a great storm, we feel that the wind is exerting a force on us. When we suspend a heavy block from a rope, the rope holds the block just as a man can hold it in air. When we comb our dry hair and bring the comb close to small pieces of paper, the pieces jump to the comb. The comb has attracted the paper pieces i.e. the comb has exerted force on the pieces. When a cork is dipped in water it comes to the surface; if we want to keep it inside water, we have to push it downward. We say that water exerts a force on the cork in the upward direction.

Force is an interaction between two objects. Force is exerted by an object A on another object B, for any force you may ask two questions, (i) who exerted this force and (ii) on which object was this force exerted ? Thus, when a block is kept on a table, the table exerts a force on the block to hold it.

Force is a vector quantity and if more than one force act on a particle we can find the resultant force using the laws of vector addition.

 

The various types of forces in nature can be grouped in four categories :

(a) Gravitational,         (b) Electromagnetic,

(c) Nuclear          and   (d) Weak. 

 (a) GRAVITATIONAL FORCE

Any two bodies attract each other by virtue of their masses. The force of attraction between two point masses is F = Gm1m2/r^2, where m1 and m2 are masses of the particles and r is the distance between them.G is a universal constant having the value 6.67 ｘ10 ^-11 n-m^2/kg^2 . To find the gravitational force on an extended body by another such body, we have to write the force on each particle of the 1st body by all the particles of the second body and then we have to sum up vectorially all the forces acting on the first body.

(b) ELECTROMAGNETIC (EM) FORCE

Over and above the gravitational force Gm1m2/r^2, the particles may exert upon each other electromagnetic forces. If two particles having charges q1 and q2 are at rest with respect to the observer, the force between them has a magnitude 

F = 1/4πદ q1q2/r^2

where દ = 8.85419 ｘ10^-12  c^2/N-m^2 is a constant. The quantity 1/4πદ is 9.0 ｘ10^9 N-m^2/c^2

Where is called coulomb force and it acts along the line joining the particles. If q1 and q2 and of same nature (both positive or both negative), the force is repulsive otherwise it is attractive. It is this force which is responsible for the attraction of small paper pieces when brought near a recently used comb. The electromagnetic force between moving charge particles is comparatively more complicated and contains term other than the coulomb force.

Ordinary matters. Each electron has 1.6 ｘ 10^-19 coulomb of negative charge and each proton has an equal amount of positive charge. In atoms, the electron are bound by the electromagnetic force acting on them due to the protons. The atoms combine to form molecules due to the electromagnetic forces. A lot of atomic and molecular phenomena result from electromagnetic forces between the subatomic particles (electrons, protons, charged mesons,etc.).

(c) NUCLEAR FORCES

Each atom contains a certain number of protons and neutrons in its nucleus. The nucleus occupies a volume of about 10^-44 m^3 whereas the atom itself has a volume of about 10^-23m^3. Thus, the nucleus occupies only 1/10^21 of  the volume of the atom. Yet it contains about 99.98% of the mass of the atom. The atomic nucleus of a non-radioactive electrons are removed from a helium atom, we get the bare nucleus of helium which is called an alpha particle. The alpha particle is a stable object and once created it can remain intact until it is not made to interact with other objects.

An alpha particle contains two protons and two neutrons. The protons will repel each other due to the Coulomb force and will try to break the nucleus. Neutrons will be silent spectators in this electro-magnetic drama (Remember, neutron is an uncharged particle). Then, why does the coulomb force fail to break the nucleus ? Can it be the gravitational attractive force which keeps the nucleus bound ? All the protons and the neutrons will take part in this attraction, but if calculated, the gravitational attraction will trun out to be totally negligible as compared to the coulomb repulsion.

In fact, a a third kind of force, altogether different and over and above the gravitational and electromagnetic force, is operating here. These forces are called Nuclear force and are exerted only if the interacting particles are protons or neutrons or both.

(d) WEAK FORCES

Yet another kind of force is encountered when reactions involving protons,electrons and neutrons take place. A neutron can change itself into a proton and simultaneously emit an electron and a particle called antinutrino. This is called β(negative) decay. Never think that a neutron is made up of a proton, an electron and an antineutrino. A proton can also change into neutron and simultaneously emit a positron ( and a neutrino ). This is called β (positive) decay. The force responsible for these changes are different from gravitational, electro-magnetic or nuclear force. Such force are called weak force. The range of weak force is very small, in fact much smaller than size of a proton or a neutron. Thus its effect is experienced inside such particles only.