Inelastic Collision
Balls stick together — momentum conserved, KE lost to heat/deformation.
Key Notes
Inelastic collision: momentum is conserved, but KE is NOT — some converts to heat, sound, deformation.
PERFECTLY inelastic: objects STICK TOGETHER. Maximum KE loss.
Common cases: cars colliding (crumple), bullet embedding in a block, clay balls sticking, two trains coupling.
Perfectly inelastic 1D: m₁u₁ + m₂u₂ = (m₁ + m₂)·v_common.
KE loss in perfectly inelastic: ΔK = ½·(m₁m₂/(m₁+m₂))·(u₁−u₂)² — reduced-mass times relative speed squared.
Real-world collisions are MOSTLY inelastic — perfectly elastic is rare in macroscopic objects.
Coefficient of restitution e: e = 1 for elastic, e = 0 for perfectly inelastic, 0 < e < 1 for partially inelastic.
Used in: crash-test physics, billiards (partial inelasticity), bullets, ballistics pendulum.
Formulas
Momentum conservation
Always holds.
Perfectly inelastic (1D)
Both bodies move together after collision.
Kinetic energy loss
Maximum loss in perfectly inelastic; vanishes for elastic.
Ballistic pendulum
Bullet (m) embeds in block (M); block rises h. Solve for bullet's speed.
Important Points
Momentum ALWAYS conserved in any isolated collision; KE not.
In perfectly inelastic: maximum KE goes to heat / deformation.
Ballistic pendulum: classic method for measuring bullet speeds.
Crumple zones in cars EXTEND collision time ⇒ reduce force, but body deformation still loses KE.
Energy 'lost' to deformation = energy that goes into binding objects together permanently.
Coefficient of restitution e measures elasticity: e = relative speed of separation / relative speed of approach.
Inelastic Collision notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.