And such interactions are described by forces. Newton's Second Law Galileo and Newton realized that an object's velocity does not change unless its interactions with its environment result in a net force. Forces cause acceleration. All that matters is how strong is the net force and in what direction it acts. We know from experience that the answer depends on the object.
Different objects resist changes in their motion differently.
You have more trouble diverting a truck from its course as compared to a fly. We say that objects have inertia , a term used to describe their tendency to resist changes. Once we choose our system, we must consider all the different forces acting on its parts, calculate their vector sum, and apply Newton's Second law with mass m equal to the sum of the masses of all the objects in the system. This can be derived from Newton's second law. We can describe the pull of gravity by a vector force pointing toward the earth, which we call the weight.