Conservation of momentum
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
P = mv conserved in absence of external force; elastic: KE conserved; inelastic: KE partially lost; perfectly inelastic: objects stick.
Conservation of momentum in plain English
Momentum is 'how hard something is to stop,' and it depends on both mass and speed: p = mv (a heavy truck and a fast bullet both have lots of it). The big rule is that in a closed system — like two things bumping into each other with no outside push — the TOTAL momentum before a collision equals the total after; it just gets shared around. What can differ is the energy of motion (kinetic energy, KE): in an 'elastic' crash it's fully preserved (think bouncy pool balls), in an 'inelastic' crash some is lost to heat and crunching (think a car fender-bender), and in a 'perfectly inelastic' crash the objects stick together and travel as one (think two lumps of clay smacking and clumping).
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