# Why Newton’s 2nd Law is called real law of motion? | Physics Grade XI

### Real law of motion

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#### Viva Voice Questions # Why Newton’s Second Law is called real law of motion?

Q. Why Newton's Second Law is called the real law of motion?

Ans: Newton's Second law of motion is called real law of motion because all the laws can be derived and are contained in this law.

Proof:

We know, F = m. a,

Where f is the external force applied on a body, ‘m’ is the mass, and ‘a’ is the acceleration produced due to external force.

If no external force acts on a body, then F = 0, since f = ma, a = 0 because mass of a body cannot be 0.

This means an object at rest will always be at rest and object moving with uniform motion in a straight line will continue to do so, which is the first law.

To prove that third law is also contained in the second law of motion, we take two bodies A and B moving along the same straight line. As a result of a collision, their momentum will change. Let t be the time of impact, then change in linear momentum of A is

PA = FA x t

Similarly, change in momentum of B is

PB = FB x t

So, the total change in momentum of A and B is,

P = pA + pB

= FA x t + FB x t

If no force acts then, P = 0

Or, FA x t + FB x t = 0

Or, FA x t = - FB x t

Or FA = -FB, which is newton’s third law of motion.

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