Stable Interactions Required 
According to Newton's Laws of Motion, a system experiencing constant acceleration is necessarily experiencing a constant net interaction. Thus, this model can only be applied when the interactions are fairly steady. It is important to remember that a constant acceleration must have a constant direction as well as a constant size. Thus, if the forces on a system are clearly changing direction during the motion being analyzed, it is not reasonable to apply the One-Dimensional Motion with Constant Acceleration model.