3.1.4—Force as rate of change of momentum

The general law is F=dp/dt. For constant mass it becomes F=ma, but variable-mass systems require momentum flux to be considered explicitly.

Choose the system boundary and direction before differentiating momentum. The force is the resultant external force on that system.

A constant 12 N force acting on a 3 kg object produces dp/dt=12 kg m s⁻² and, at fixed mass, acceleration 4 m s⁻².

F=ma is a special constant-mass form; force is not simply momentum divided by time unless the change is defined correctly.