Newton’s first law
Following Aristotle, people used to think for many centuries that we need to make an effort to uphold motion. This is a quite reasonable assumption if we consider that we are surrounded by friction, which has to be overcome to sustain motion. Yet this kind of thinking did not allow a powerful description of the causes of motion to emerge.
It took Newton's genius to subvert this notion and realise that objects, by themselves, tend to retain their state of motion, and it takes an effort not to sustain but to change this state of motion. We perceive the need to make an effort to uphold motion because friction acts to change it and we need to counter this effect for the object to remain in the same state of motion. This realisation is what we call Newton's first law, and this is the conceptual foundation of Newtonian dynamics.
Newton’s first law
Every object continues its state of rest or its motion with constant velocity unless it is compelled to change its state of motion by forces exerted upon it by the environment.
This law is also referred to as the law of inertia.
Inertia
Inertia is the resistance of an object to changes in its state of motion. The physical quantity that quantifies inertia is mass.
Mass
Mass is the property of an object that specifies how much resistance the object has to changes in its velocity. Mass is the measure of the inertia of an object, that is, of its resistance to changes in its state of motion. Mass is an inherent property of an object and is independent of the object's surroundings (such as the gravitational field the object is in).
Mass is a scalar quantity.
The SI unit of mass is the kilogramme, which is an SI base unit:
\[\left[m\right] = 1\,\mathrm{kg}.\]
Note: mass is different to weight. Weight is the magnitude of the force that must be applied to an object in order to support it (ie, hold it at rest) in a gravitational field, whilst mass, as we have mentioned, is independent of the gravitational field.
Examples for Newton’s first law:
A ball put on the table stays at rest – it retains its state of motion –, but if we flick the ball, it will start to move, which means it changes its state of motion. An ice hockey puck sliding on ice moves with constant velocity (retains its state of motion) until it meets the wall or the stick of a player (a force which causes the state of motion to change).