In classical Newtonian physics, the concept of mass is fundamental. Mass is the measure of an object's inertia and its resistance to changes in motion. According to Newton's second law of motion, F = ma, the mass of an object determines how it responds to an applied force, resulting in an acceleration.
However, in modern physics, specifically in the theory of relativity, it is recognized that massless particles do exist. Massless particles, such as photons (particles of light), gluons, and gravitons, are an integral part of the standard model of particle physics. These particles travel at the speed of light and have no rest mass.
It is important to note that the behavior of massless particles, such as their interactions and motion, is described by different physical theories, including quantum mechanics and the theory of relativity, rather than classical Newtonian mechanics. These theories provide a more accurate and comprehensive understanding of the behavior of particles, both with and without mass.