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Force

Force is an external agency which produces or tends to produce, destroys or tends to destroy the motion of a body.
https://youtu.be/QOKdPtnePeg
  a force while acting on a body  may
(a) change the motion of a body
(b) retard the motion of a body
(c) balance the forces already acting on a body, and
(d) give rise to the internal stresses in a body
           
Force is usually expressed in Newtons (briefly written as N). it may be noted that,
                          1kgf=9.81N




  In order to determine the effect of a force acting on a body, we must know the magnitude of the force, the line of action of the force, the nature of the force i.e, pull or push and the point at which the force is acting.

Moment of a force: the turning effect produced by a force on the body on which it acts. The moment of a force is equal to the product of the force and the perpendicular distance between the line of action of the force and the point about which the moment is required.
Mathematically, the moment of a force F about point O as shown in figure
                  = F × d 





Resultant force:
Resultant force is a single force which produces the same effect as produced by the given set of forces acting on a body. Resultant force is determined by the three laws of Forces.
1.Parallelogram law of forces:

If two forces, acting on a particle simultaneously be represented in magnitude and direction by the two adjacent sides of a parallelogram then their resultant may be represented in magnitude and direction by the diagonal of a parallelogram which passes through their point of intersection. 
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