According to Newton’s first law, if a body is at rest or moving in a straight line at a constant speed, it will continue to move at that speed or stay at rest until acted upon by a force. In fact, according to classical Newtonian mechanics, there is no significant difference between being at rest and moving uniformly in a straight line; they can both be thought of as states of motion experienced by different observers, one of whom moves at the same speed as the particle and the other of whom moves at a constant speed in relation to the particle. The rule of inertia is the name given to this premise.
Galileo Galilei first proposed the law of inertia for Earth’s horizontal motion, and René Descartes subsequently generalized it. Although the basic premise and starting point of classical mechanics is the principle of inertia, the untrained eye finds it to be less than immediately clear. In both everyday life and Aristotelian mechanics, things that are not being pushed have a tendency to come to rest. Galileo derived the law of inertia from his studies involving balls rolling down incline planes.
Galileo had to explain how it was possible that, if the Earth is truly spinning on its axis and orbiting the Sun, we do not feel that motion. To do this, he had to explain the principle of inertia, which was essential to his main scientific job. The response is supported by the concept of inertia, which states that because we move with the Earth and have a tendency to keep moving, Earth appears to be at rest to us. As a result, the concept of inertia was once a major topic of debate in science and was far from being a statement of the obvious. It was possible to correctly account for the minor deviations from this picture brought on by the fact that the motion of the Earth’s surface is not uniform motion in a straight line by the time Newton had worked out all the details (the effects of rotational motion are discussed below). The common observation that bodies that are not pushed tend to come to rest is explained by the Newtonian formulation as the result of unbalanced forces operating on them, like friction and air resistance.
