Question

Deduce Newton’s first law from the second law.

Answer 1

In order to solve this question, you must be aware of Newton’s Laws of motion. The first law states that the object will not change its state unless a force acts on it. The second law states that the force on an object is equal to its mass times its acceleration. The third law states that, when two objects interact, they apply equal and opposite forces on each other.

Complete answer:
According to Newton’s Second law,
$F = ma$
F = m$\dfrac{{(v - u)}}{t}$
$F = m\dfrac{{(v - u)}}{t}$
If, $F = 0$
Then, $v = u$
i.e. the object continues to move with uniform velocity if no force is applied.
If, $F = 0$ and $u = 0$, then, v is also 0
i.e. the object will stay at rest.
According to Newton’s First law of motion, an object at rest or uniform motion tends to remain at rest or in uniform motion unless an unbalanced force acts on it.
Hence proved.

Note: If no net force is acting on the object, then the object won’t change its state of motion. It will continue to stay at rest or in motion and Newton's law of motion will be valid.