## Defination of Force:-

Force may be defined as an agent which produces or tends to produce, destroy or tends to destroy the motion of a body.

Mathematically force P= m.a (where m= mass and a=acceleration)

Action of force: A force while acting on a body may
(a) Change the motion of the body.
B. Retard the motion of the body
C. Balance the forces already acting on a body, and
D. Give rise to internal stresses in a body.
In order to determine the effects of a force acting on a body, we must know the following characteristics of force:

• The magnitude of the force
• The line of action of the force
• The nature of the force , i.e. push or pull, and
• The point at which the force is acting.

Unit of force: In MKS system the force is expressed in kgf (kilogram-force) and in SI system of unit, the force is expressed in newtons (N).
Mks unit of force=kgf
SI unit of force= Newton’s(N)
CGS unit of force= dyne
1 kgf= 9.81 N
1 N = 10^5 dyne
Resultant Force: It is a single force which produces the same effect as produced by all the given forces acting on a body. The resultant force may be determined by the following three law of forces:

Parallelogram law of forces: It states that if two forces, acting simultaneously on a particle, be represented in magnitude and direction by the two adjacent side of a parallelogram, then their resultant may be represent in magnitude and direction by the diagonal of the parallelogram which passes through their point of intersection.

For example, let us consider two forces P and Q acting at angle  Ø at a point O as shown in figure. The resultant force is given by,

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