Why doesn't Newton's second law apply in situations where mass varies?

Newton's second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass, which is expressed in the formula F = ma. This law assumes that both mass and force are constant.

In situations where mass varies, such as a rocket propelling itself by expelling gas, the mass of the rocket decreases over time as it burns fuel. If the mass isn’t constant, it becomes challenging to apply the equation in a straightforward manner. The changes in mass can lead to variations in the acceleration that cannot be accurately predicted by simply applying the law as it stands.

When the mass changes, defining the net force and resultant acceleration becomes complex, as the law does not account for that dynamic situation. Therefore, the inconsistency in the forces due to the variable mass implies that the simple relationship of F = ma may not hold during that period of change.