Why is the equation F=ma considered non-relativistic?

The equation F=ma is considered non-relativistic because it does not accommodate the effects that occur when an object's speed approaches the speed of light. In the realm of classical mechanics, as described by Newton, mass is treated as a constant, regardless of the object's speed. However, Einstein's theory of relativity shows that as an object approaches the speed of light, the mass effectively increases—this is often referred to as relativistic mass.

In relativity, the force must take into account this change in mass, which alters how we understand motion at high velocities. As such, F=ma does not hold true under these conditions, making it unsuitable for describing phenomena occurring at relativistic speeds.