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Force:

Force is an external effort in the form of push or pull.
1. It produces or tries to initiates the motion of a static body.
2. It stops or tries to stop a moving body.
3. It changes or tries to change the direction of motion of a moving body.
Force has both magnitude and direction. So, it is a vector quantity.

Units of force:

The SI unit of force is newton ($$N$$), and in the C.G.S. system, its unit is dyne.

$$1\ N$$$$ewton$$ ($$N$$):

The amount of force needed for a body of mass $$1\ kg$$ produces an acceleration of $$1$$ $m{s}^{-2}$,

$$1$$ $$N$$ $$=$$ $$1$$ $$kg$$$m{s}^{-2}$

$$1\ dyne$$:

The amount of force needed for a body of mass $$1\ gram$$ produces an acceleration of $$1$$ $\mathit{cm}{s}^{-2}$,

$$1$$ $$dyne$$ $$=$$ $$1\ g$$ $\mathit{cm}{s}^{-2}$, also

$$1$$ $$N$$ $$=$$ $$10^5$$ $$dyne$$

Unit force:

The amount of force needed to produce an acceleration of $$1$$ $m{s}^{-2}$in a body of mass $$1$$ $$kg$$ is known as ‘unit force’.

Gravitational unit of force:

In the S.I. system of units, the gravitational unit of force is kilogram-force, represented by $$kg f$$. In the C.G.S. system, its unit is $$gram force$$, represented by $$g f$$.

$$1$$ $$kg f$$ $$=$$ $$1$$ $$kg \times 9.8$$  $$m s^{-2}$$ $$=$$  $$9.8$$ $$N$$

$$1$$ $$g f$$ $$=$$ $$1$$ $$g \times 980$$ $$cm s^{-2}$$ $$=$$ $$980$$ $$dyne$$

$$1$$ $$kg f$$ $$=$$ $$1000$$ $$g \times 980$$ $$cm s^{-2}$$ $$=$$ $$980 \times 10^{3}$$  $$dyne$$