# How do you apply each of Newton's laws to specific scenarios?

The 3 Newton's laws of motion are easy to be understood but examples citing which law caused it are so confusing - say a bullet fired from a gun - many people state it cause is 3rd Law - but I see all 3 laws are applied:

First law causes the bullet change its state of inertia - second cause the bullet gained momentum and third cause as an action of gun recoil it got fired.

Similarly in all examples I see it - a bus started to move and we lean back - here first law is cause we changed in our state of motion - second law is cause force mass and speed are related here - more mass more force then more speed of us moving and third law is cause a force exerted us and a reaction force will be there

In all I really get so confused on reading example and relating Newton's law that I cannot just get them correct at times.

Is there a way to relate Newton's Laws of Motion to experiments - means it happened due to # law of Newtons laws of motion?

• Can you please further explain what you exactly want to say. – SarGe May 13 '20 at 15:54
• Any event, like firing a gun, has different aspects of it described by each of the three laws. In a way, Nature doesn't care that we have 3 instead of 2 or 5 or 10 laws; nature just does its thing. Newton found 3 sets of calculations which describe what nature just did. – simon at rcl May 13 '20 at 16:01
• Since the whole point of Newton's laws is that together they form a reasonably complete theory of mechanics, they are always in effect simultaneously. I'm not really sure why you'd want to isolate specific laws to specific experiments, nor what the source of these examples is that you're apparently reading. Your question would be clearer if you directly included at least one of the examples you're talking about. – ACuriousMind May 13 '20 at 16:10

## 1 Answer

You're essentially right that all the three laws of Newton are at play in all mechanical phenomena. Otherwise, they wouldn't be laws, would they? However, notice that the first and the second laws are more fundamental than the third law--so you can have situations where the third law is violated but the second and the first laws are not. This kind of situations arise in the electrodynamical context, see, for example, Introduction to Electrodynamics (4th Edition), Griffiths, D., Section 8.2.1. But, so long as the forces are such that the third law is also followed, in all mechanical phenomena, all three laws play a role.

However, you can ask the question as to which aspect of a mechanical phenomenon is associated with which law of Newton. This might get somewhat subjective but often not. For example, what is the physical principle that is crucial to the working of skiing? I'd argue it's the third law of Newton (momentum conservation, actually) because that's what makes sure that when you apply a force on the ground, the ground will apply a force back on you. The first and the second laws are, of course, at work too. But with respect to the aspect of the phenomenon we are interested in, i.e., the aspect that we apply a force on the ground to get ourselves accelerated, corresponds most closely with the third law of Newton.