Many people term this law simply as Newton’s first law, which is inappropriate. Because if not properly mentioned as 1st law of motion, it’s not exactly clear whether Newton’s law of gravitation, or his law of viscosity or any other of his laws is referred to. So the law under discussion should be termed as Newton’s 1st law of motion instead of simply Newton’s 1st law.
The law is – If no external force is applied on a body its velocity or lack of motion isn’t hampered. In other words, a body’s static situation or fixed velocity motion is hampered or altered if and only if an external force is applied on it.
While no external force is applied on a body or the resultant of the applied forces is zero, Newton’s 1st law of motion applies for it. Imagine a place in space where gravitational effect is very negligible (ideally zero), because all the planets, asteroids, stars and other celestial bodies are so far away from here. Now think of a comet in a place like this. It will run at a steady velocity owing to the fact that no external force is applied on it. Fixed or steady velocity means steady speed in a particular direction. So in this case Newton’s first law of motion applies.
Now think of a still toy kept on the floor. Although a number of forces are applying on it (weight of the toy and NORMAL reaction force on it from the floor), the forces cancel each other and hence their resultant is zero. So Newton’s first law of motion applies for this toy too.
Idea of Force and Inertia: from Newton’s 1st law of motion
The idea of force and inertia are obtained from Newton’s 1st law of motion. The abstract noun, absence of which causes a body to remain still or moving in fixed velocity, is called Force. A force can cause a stationery object to move, or cause a moving object to change its velocity. Apart from these, a force when applied on an object can change the shape, the size and/or the volume of the object. Change in velocity means change in the velocity’s magnitude, or its direction or both.
In addition to force, the idea of Inertia is also obtained from Newton’s 1st law of motion. Inertia is the property of a body because of which it shows reluctance when attempts are taken to change the stationery condition or the velocity of the body. The more is the inertia of a body, the greater reluctance it shows when attempts are made to change the state of motion of the body.
There are two types of inertia: stationery/static inertia and motion inertia. The property of a stationery object because of which it shows reluctance to change its stationery condition or stillness, is called its static inertia. And the property of a moving object because of which it shows reluctance to change its velocity is called its its motion inertia. Static inertia and motion inertia of a body are proportional respectively to its mass and momentum. The heavier a stationery object is, the tougher it will be to cause it to move in a particular velocity within a certain time interval. (Remember that in case of a stationery object the velocity is thought to be zero). On the other hand, the greater the momentum of a moving body is, the tougher it will be to change its velocity by a certain amount within a certain time interval. (Momentum is a property of a moving body which is measured as the product of its mass and its velocity).
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