Summarizing Conservation of Momentum

Summary

• The conservation of momentum principle is written
  \({\mathbf{p}}_{\text{tot}}=\text{constant}\)
  or
  \({\mathbf{\text{p}}}_{\text{tot}}={\mathbf{\text{p}}\prime }_{\text{tot}}\phantom{\rule{0.25em}{0ex}}\phantom{\rule{0.25em}{0ex}}\left(\text{isolated system}\right),\)
  \({\mathbf{p}}_{\text{tot}}\) is the initial total momentum and \({\mathbf{\text{p}}\prime }_{\text{tot}}\) is the total momentum some time later.
• An isolated system is defined to be one for which the net external force is zero \(\left({\mathbf{\text{F}}}_{\text{net}}=0\right)\text{.}\)
• During projectile motion and where air resistance is negligible, momentum is conserved in the horizontal direction because horizontal forces are zero.
• Conservation of momentum applies only when the net external force is zero.
• The conservation of momentum principle is valid when considering systems of particles.

Glossary

conservation of momentum principle

when the net external force is zero, the total momentum of the system is conserved or constant

isolated system

a system in which the net external force is zero

quark

fundamental constituent of matter and an elementary particle