Definition

Newton's second law says that a bigger push makes something speed up faster, and a heavier object is harder to speed up; force, mass, and acceleration are all connected. Precisely, the acceleration of an object equals the net force divided by its mass, $F_{\text{net}} = ma$ or $a = F_{\text{net}}/m$, so acceleration is directly proportional to net force, inversely proportional to mass, and points in the direction of the net force, with units of newtons, kilograms, and meters per second squared. In vector form the law is $\vec{F}_{\text{net}} = m\vec{a}$, and more generally $\vec{F}_{\text{net}} = d\vec{p}/dt$, the rate of change of momentum, which handles variable-mass systems such as rockets. Applied per axis, it yields coupled equations of motion whose solutions describe trajectories. Combined with the third law it leads to conservation of momentum for isolated systems.