Elastic and inelastic collisions
Momentum conservation and kinetic-energy conservation are different.
Key knowledge
When external impulse is negligible, total momentum is conserved through a Newtonian collision. An elastic collision also preserves total translational kinetic energy; an inelastic collision does not. In a perfectly inelastic collision two bodies stick and have common final velocity (m₁v₁+m₂v₂)/(m₁+m₂). Their translational kinetic-energy decrease becomes other forms of energy rather than destroying total energy.
Values and equations
v_f=(m₁v₁+m₂v₂)/(m₁+m₂); K_i−K_f=½[m₁m₂/(m₁+m₂)]|v₁−v₂|²
- v₁,v₂,v_f
- Initial and common final vector velocities m s⁻¹
- m₁,m₂
- Fixed body masses kg
- K_i,K_f
- Initial/final translational kinetic energy in the sticking model J
Conditions and scope
- Use an inertial frame, fixed positive masses and negligible external impulse during the collision.
- The displayed sticking model concerns point-like bodies or translation only; rotation, internal excitation and energy release need their own accounting. Inelastic is not universally synonymous with kinetic-energy loss in an exothermic interaction.
Sources · 1
References
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Evidence and review
Read §9.4 elastic/inelastic/perfectly-inelastic definitions and sticking momentum equation. The kinetic-loss expression is a disclosed algebraic consequence for positive masses; do not extend the ordinary dissipative examples to exothermic particle reactions.
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Entry details
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- foundations-refresh.1
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