In relativistic physics, where objects move at speeds close to the speed of light, the principles of classical mechanics, including the kinematic equations, no longer apply. This is because, at such high velocities, time dilation and length contraction – effects predicted by Einstein's theory of relativity – become significant. As a result, the classical definitions of time and space, and consequently velocity and acceleration, alter fundamentally. The kinematic equations are based on Newtonian mechanics, which assumes that time and space are absolute and do not account for the relativistic effects. Therefore, in scenarios involving relativistic speeds, the equations of special relativity must be used instead.

The assumption of treating an object as a point mass is invalid in scenarios where the size, shape, or rotational dynamics of the object significantly influence its motion. For example, in situations where an object spins, tumbles, or has parts that move relative to one another, the internal dynamics and distribution of mass become important. In aerodynamics, the shape and size of objects like aircraft significantly affect air resistance, making the point mass approximation unsuitable. Similarly, in cases involving long objects moving through mediums (like a rod moving end-first through water), the object's length and interaction with the medium cannot be ignored.

In circular motion, even if the speed of an object remains constant, its velocity is constantly changing due to the continuous change in direction. Uniformly accelerated motion applies to circular motion in the sense that there is a constant rate of change of velocity, but this change is in direction rather than in speed. The acceleration in circular motion is termed centripetal acceleration, which is always directed towards the centre of the circle. Although the kinematic equations are not directly applicable in their standard form due to the multidimensional nature of circular motion, the concept of constant acceleration (in terms of changing direction at a constant rate) still holds. This demonstrates the versatile nature of acceleration in different motion contexts.

Air resistance and friction introduce non-uniform forces that can alter an object's motion, affecting the applicability of the kinematic equations. These equations assume uniform acceleration, which means they are ideal for scenarios where the only force acting is constant, like gravity in free fall. However, air resistance and friction are forces that vary with factors like speed, surface texture, and the medium through which the object is moving. They cause the acceleration to change, making it non-uniform. In real-world scenarios where air resistance and friction are significant, these equations become less accurate, and more complex models that take these forces into account are required.

The kinematic equations derived for uniformly accelerated motion are primarily applicable to one-dimensional motion. This means they are most effective when dealing with motion along a straight line, where the direction of motion and acceleration do not change. In two or three dimensions, such as in projectile motion or circular motion, the situation becomes more complex. In these cases, the motion can be resolved into components, typically using the x and y axes for two-dimensional motion. The kinematic equations can then be applied separately to each component. For example, in projectile motion, the horizontal and vertical motions are treated independently, with acceleration due to gravity affecting only the vertical component. However, for a complete analysis of two or three-dimensional motion, additional principles and equations from vector mechanics are necessary.