Elastic Collisions
Overview
Both momentum AND kinetic energy are conserved. Objects bounce off each other. Total KE before = Total KE after. This is an idealised case (perfectly elastic). In practice, truly elastic collisions occur between atomic/subatomic particles. At the macroscopic level, some energy is always lost.
Elastic vs. inelastic collisions
In an ELASTIC collision, both momentum AND kinetic energy are conserved. Objects bounce off without any deformation or heat generated. In an INELASTIC collision, momentum is conserved but kinetic energy is NOT (some KE is converted to heat, sound, or deformation). In a PERFECTLY inelastic collision, the objects stick together — maximum KE is lost (but momentum is still conserved).
Special case: elastic collision of equal masses
When two objects of equal mass collide elastically and one is stationary, the moving one stops and the stationary one moves off with the same speed. This is seen with Newton's cradle. For unequal masses: if a light object hits a very heavy stationary one, it bounces back with nearly the same speed. If a heavy object hits a stationary light one, it barely slows down and the light one moves off at about twice the incoming speed.
- ⚠Assuming all collisions are elastic — most real collisions are inelastic
- ⚠Forgetting to use v² (not v) in the KE formula: KE = ½mv²
- ⚠Thinking that because momentum is conserved, energy must also be conserved