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Force | What is force? | Classification of Forces | unit of force

Force  Aristotle famously represented anything that causes an object to undergo "unnatural motion".  Sir Isaac Newton was one of t...

Force
Force 

Aristotle famously represented anything that causes an object to undergo "unnatural motion".  Sir Isaac Newton was one of the first scientists to study gravity and force.  Any kind of force is just a push or a pull.  It can be described as a push or pull on an object.

Table of Content

  • What is force?
  • Classification of Forces
  • Units of Force
  • Solved Force Examples

What is force?

Force can be defined as the physical cause that tends to change the resting position or uniform motion or to change the shape and size of the body.


Classification of Forces

Forces are classified as contact and non - contact forces.  
  • Forces can be applied on a body by making physical contact or otherwise.   
  • Forces which act on a body through physical contact are contact forces.   
  • Forces which act on a body without making any physical contact with the body are known as non - contact forces.  

Based on their nature, forces can also be classified as: 
  1. Mechanical force  
  2. Magnetic force 
  3. Gravitational force 
  4. Electric force 
  5. Nuclear force

Mechanical force 

To hit a nail into a wall, we use a hammer.  The force exerted by the hammer on the nail rives into the wall.  Thus, in this case, the force makes a physical contact and displaces the mail Such a force is called contact force or in this case, it can also be called mechanical force.

Magnetic Force 

Place a small iron piece near a magnet.  The iron piece moves wards the magnet.  This force applied by the magnet is called magnetic force.   
Magnetic force is a non - contact force, since the iron piece experiences a force though there is no physical contact.

Magnet attracting an Iron piece
FIGURE 2 Magnet attracting an Iron piece


 


Gravitational Force  

A ball thrown vertically upwards returns to the ground after some time . This is on account of the attractive force the Earth exerts on the ball . Similarly , the Earth also exerts an attractive force on celestial bodies such as the moon , the sun , the planets , etc. This force of attraction between any two masses is known as gravitational force.

Electric Force 


Charged glass rod attracting a paper
Charged glass rod attracting a paper

Glass rod attracting a paper When a glass rod is rubbed with wool , the glass rod acquires electric charge . Now , when the glass rod is brought near some pieces of paper , they cling to the glass rod . This is due to the electric force of the glass rod.

Nuclear Force   

A nuclear force is a force that exists only if the interacting particles are protons or neutrons or both . It is different from gravitational and electromagnetic forces.


Units of Force 

Absolute unit of force
If m = 1 unit, a = 1 unit, then the value of F = 1 unit

C.G.S.  unit of force is 'dyne' and S.I.  unit of force is 'newton' (N).  

Dyne: It is the force which when acting on a body of mass one gram produces in it an acceleration of 1 cm s^-2 in its direction.

Newton: It is the force which when acting on a body of mass 1 kg produces in it an acceleration of 1 m s^-2 in its direction.  

Therefore 
 1 newton (N) = 1 kg × 1 m s^-2

Solved Force Examples

EXAMPLE 1
What is the force exerted by a bullock in pulling a cart of mass 100 kg and accelerating at a rate of 1.5 m s^-2?
SOLUTION  
F=?  
m = 100 kg  
a = 1.5 m s^-2  
From Newton's second law,  
F= ma 
Therefore 
F= 100 kg x 1.5 m s^-2 = 150 N 

EXAMPLE 2 
An object of mass 10 kgis moving with an initial velocity of 10 ms^-1. A constant force acts for 4s on the object giving it a speed of 2 m s^-1 in the opposite direction. Find the acceleration and force.
SOLUTION  
m=10 kg 
u= 10 m s^-1 
Let the final velocity in the opposite direction be v. 
t= 4s  
v= -2 m s^-1 
a=?
F=? 

A=v-u/t(From the equations of motion,v=u+at) 
    
a=-2-10/4=-12/4=-3 s^-2

The negative sign shows that it is retardation.  
From Newton's second law,  
F= ma = 10×-3=-30 N  
Retarding force = 30 N.