reply to brews ohare brews i am fully aware of the difference between kinematics and kinetics this aticle doesn't need to involve that difference you quoted above an expression for acceleration that expression is correct i am not disagreeing with it but it tells us absolutely nothing that would enable us to solve a motion problem to solve a problem in motion we need an input force if it only kinematics we still need an input acceleration we always need some kinds of contraints or known accelerations before we can determine a trajectory the solution to your general acceleration equation above is anything