to the presence of different objects. The third law means that all forces are interactions between different bodies. and thus that there is no such thing as a unidirectional force or a force that acts on only one body.

In a system composed of object 1 and object 2, the net force on the system due to their mutual interactions is zero:

More generally, in a closed system of particles, all internal forces are balanced. The particles may accelerate with respect to each other but the center of mass

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