# First Law Inertia > An object at rest will stay at rest and an object in motion will stay in motion, unless affected by an external force >$ \sum\vec{F}=0 $ # Second Law Force > The sum of forces on an object is equal to change in momentum per unit time > > $ > \sum\vec{F}=\frac{d(m\vec{v})}{dt}=m\frac{d\vec{v}}{dt}=ma > $ # Third Law Action and Reaction > A force by one object on another object has an equal and opposite reaction on the first object