What is Linear Momentum?
Linear momentum is defined as the product of mass of the particle and its velocity, it is a vector quantity.
\(\overrightarrow{p}=m\overrightarrow{v}\)From newton’s second law of motion, for a fixed mass particle
\(\overrightarrow{F}=m\overrightarrow{a}=m\frac{d\overrightarrow{v}}{dt}=\frac{d}{dt}\left( m\overrightarrow{v} \right)=\frac{d\overrightarrow{p}}{dt}\)For a system of n particles with masses \({{m}_{1}},{{m}_{2}},{{m}_{3}},…{{m}_{n}}\)and velocities \(\overrightarrow{{{v}_{1}}},\overrightarrow{{{v}_{2}}},\overrightarrow{{{v}_{3}}},…\overrightarrow{{{v}_{n}}}\) respectively, then the net momentum of the system is
\(\overrightarrow{{{p}_{net}}}={{m}_{1}}\overrightarrow{{{v}_{1}}}+{{m}_{2}}\overrightarrow{{{v}_{2}}}+{{m}_{3}}\overrightarrow{{{v}_{3}}}+…+{{m}_{n}}\overrightarrow{{{v}_{n}}}=\overrightarrow{{{p}_{1}}}+\overrightarrow{{{p}_{2}}}+\overrightarrow{{{p}_{3}}}+…+\overrightarrow{{{p}_{n}}}\) \(\overrightarrow{{{p}_{net}}}=M\overrightarrow{{{V}_{cm}}}\)Differentiating the above expression with respect to time
\(\frac{d\overrightarrow{{{p}_{net}}}}{dt}=M\frac{d\overrightarrow{{{V}_{cm}}}}{dt}\) \(\overrightarrow{{{F}_{net}}}=M\overrightarrow{{{a}_{cm}}}\)And also
\(\overrightarrow{{{F}_{net}}}=\frac{d\overrightarrow{{{p}_{net}}}}{dt}\)The magnitude of linear momentum may be expressed in terms of kinetic energy as well
\(p=mv\) \({{p}^{2}}={{m}^{2}}{{v}^{2}}=2m\left( \frac{1}{2}m{{v}^{2}} \right)=2mK\)Law of Conservation of Momentum
If the net force acting on a body is equal to zero, then the momentum of the body remains constant. This is known as law of conservation of momentum.
\({{F}_{net}}=0\) \(\frac{d{{p}_{net}}}{dt}=0\)Therefore, pnet = 0 or pnet = constant
If the velocity of centre of mass is equal to zero, \(\left( {{v}_{cm}}=0 \right)\) then from \(\overrightarrow{{{p}_{net}}}=M\overrightarrow{{{V}_{cm}}}\) we get, \(\overrightarrow{{{p}_{net}}}=0.\)
If the velocity of centre of mass is constant (vcm = constant) then we get \(\overrightarrow{{{p}_{net}}}\) = constant.
This is known as the law of conservation of linear momentum of the system of particles.