# Find out the solution of this problem [closed]

1. Can Newton's first law of motion be considered merely as a special case of second law of motion with $$a=0$$? If so, the first law really needed?

2. Why does the acceleration of a freely falling body not depend on the mass of the object?

• Welcome to Physics SE. Please, notice that it is highly recommended to ask well focused questions. In your case, there are two unrelated questions. Moreover, when composing a question, you'll see that some related previous questions are presented. Maybe your question has already an answer. It will reduce the chances that your question would be closed as duplicate. – GiorgioP Sep 27 at 7:56
• The first question is a duplicate of physics.stackexchange.com/q/122231/50583, the second of physics.stackexchange.com/q/11321/50583. – ACuriousMind Sep 27 at 10:25
• This post (v3) is a good example of why one should only ask 1 question per post. – Qmechanic Sep 27 at 10:27

2. The simple answer is because the weight of an object is proportional to its mass. Newton’s second law tells us that $$W =ma$$ where $$W$$ is the weight of the object, but if $$W=mg$$ then $$m$$ appears on both sides of the equation of motion and cancels out. But the really interesting question is why gravitational mass (the $$m$$ in $$W=mg$$) should be always the same as inertial mass (the $$m$$ in $$F=ma$$). It was thinking about this question that led Einstein to discover the theory of general relativity.