# Momentum > *Definition 1*: the **momentum** $\mathbf{p}$ of a particle is defined as the product of the mass and velocity of the particle > > $$ > \mathbf{p} = m \mathbf{v}, > $$ > > with $m$ the mass of the particle and $\mathbf{v}$ the velocity of the particle. For the case that $\mathbf{v}: t \to \mathbf{v}(t) \implies \mathbf{v}'(t) = \mathbf{a}(t)$ we have the following theorem. > *Theorem 1*: let $\mathbf{v}$, $\mathbf{a}$ be the velocity and acceleration of a particle respectively, if we have > > $$ > \mathbf{v}: t \to \mathbf{v}(t) \implies \forall t \in \mathbb{R}: \mathbf{v}'(t) = \mathbf{a}(t), > $$ > > then > > $$ > \mathbf{p}'(t) = \mathbf{F}(t), > $$ > > for all $t \in \mathbb{R}$. ??? note "*Proof*:" Will be added later.