**Explanation:** Both First and Second Law of Newton describe ‘force’. However, in First Law the description is qualitative (emphasis on the quality of force that it moves or stops a body) and the Second Law gives a quantitative measurement of the force, in newtons! Also we note that if the force produces acceleration in its own direction, then the velocity of an already moving body will further increase if the applied force and velocity of the body are in one direction. In such a case, the acceleration is positive. One the other hand, if the direction of the force is opposite to the direction of the velocity of the body, the applied force will slow down the velocity of the body. In such cases, the acceleration is said to be negative.

**Unit of force:** One interesting result of Newton’s Second Law is that it gives us the definition of the unit of force, newton. “The force is said to be 1 newton if it produces an acceleration of 1 ms^{-2} when applied to a body of 1 kg”.

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