# So Why do We do Physics? Part 2: The Law of Inertia

Newton’s three laws of motion are the basis of Newtonian mechanics. Newton’s first law, which is also known as the law of inertia, state that an object at rest or moving at a constant velocity will continue to do so if there are no external forces acting on the object. Newton’s second law state that an object’s acceleration is equal to the net force per unit mass applied to the object, with the acceleration in the same direction as the net force. The first law follows, in principle, from the second because the object’s velocity cannot change if there is no external force acting on the object. The law of inertia can also be seen as a special case of the conservation of momentum in the absence of collisions (which causes an external force to act on the objects under consideration). Lastly, Newton’s third law state that for every action (force), there is an equal and opposite reaction (i.e. a reaction force which is equal in magnitude but directed in the opposite direction).

A reason which is often given to explain why students continued with postgraduate studies, is due to their inertia. I used this reason also in deciding whether or not to pursue a doctoral degree: I liked the research topic I had for my masters degree and I enjoyed working with my study leader/supervisor; I could easily continue with this research topic under the same study leader for doctoral studies; I also thought that it might be easier to continue with doctoral studies while I’m already busy studying and in the habit of doing research, as it might be difficult to start a doctoral degree after having worked for some time (part time studying is rough if you already have a job and/or a family and one would need to reacquaint oneself with the subject field and its developments over the years of absence).

The existential crises (of which this blog talks about) of course comes in when you realise that the only reason you did something was due to inertia and not because you really wanted to. I luckily had some additional reasons (which I shall discuss in future blog posts) to do a doctoral degree and could willingly let Newton’s first law to run its course without applying an external force. Have you ever done something purely due to inertia? Did you regret this later on or were you glad that inertia moved you in the correct direction?

As for Newton’s third law? The ‘why?’-question is the indication of the difficulty and challenges (reaction) we experience due to the force (our efforts) we apply in making a success of our studies/work/projects/etc. But since everything is relative, maybe our efforts is the reaction to the force exerted on us by our studies/work/projects/etc…