 Force

A force describes an interaction's contribution to the rate of change of momentum. In an inertial frame, Newton's second law relates the vector sum of external forces on a particle to $d\mathbf{p}/dt$. For constant Newtonian mass, this becomes $\sum \mathbf{F}=m\mathbf{a}$.

A cart pulled along a nearly frictionless track illustrates the distinction between one applied force and the resultant. A spring scale measures the pull, while video tracking measures acceleration; friction and track slope must also enter the balance. Opposing pulls can cancel, leaving zero acceleration even though each pull is nonzero.

A force's point or line of application also matters for an extended body, because it determines the torque. The same resultant force can produce different rotational motion when applied at different locations. A free-body diagram makes both the chosen body and its external interactions explicit.

Force is a fundamental concept in physics that refers to an interaction that causes an object to undergo a change in speed, direction, or shape. It can be thought of as a push or pull applied to an object. Force is a vector quantity, which means it has both magnitude and direction.