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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 \(1\) ms2,
\(1\) \(N\) \(=\) \(1\) \(kg\)ms2
\(1 dyne\):
The amount of force needed for a body of mass \(1 gram\) produces an acceleration of \(1\) cms2,

\(1\) \(dyne\) \(=\) \(1 g\) cms2, also
\(1\) \(N\) \(=\) \(105\) \(dyne\)
Unit force:
The amount of force needed to produce an acceleration of \(1\) ms2in 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 \(g f\).
\(1\) \(kg f\) \(=\) \(1\) \(kg \times 9.8\)  \(m s^{-2}\) \(=\)  \(9.8\) \(N\);
\(1\) \(g f\) \(=\) \(1\) \(g \times 980\) \(cm s^{-2}\) \(=\) \(980\) \(dyne\)
\(1\) \(kg f\) \(=\) \(1000\) \(g \times 980\) \(cm s^{-2}\) \(=\) \(980 \times 10^{3}\)  \(dyne\)