Question

Under what circumstances is Newton's first law same as Newton's second law.

Answer 1

Step 1: Given

Newton's first law same as Newton's second law

Step 2: Explaining circumstances

Newton's second law,
$F=ma,a=\frac{\text{change in velocity}}{\text{time}}$
$F=m\left(\frac{v-u}{t}\right)$

$F=0,a=0v=u$

$\frac{v-u}{t}=0v=u$
It means that a body will remain in rest or in uniform motion along a straight line if no external force act on it

Hence, we can say if the force applied on a body is zero, Newton's first law becomes a special case of Newton's second law.