Impedance mismatch causes partial reflection of energy between unequal masses.

Evidence

Complete transfer of momentum does not happen when a nickel collides with a penny at rest, nor when a penny collides with a nickel at rest. The energy of the first, moving coin is only partially transferred to the second. If we make a line of coins, each coin becomes an agent of transfer of energy from left to right, if the first coin was traveling in that direction. A coin that is much heavier or lighter than its neighbors will impede the transfer of energy. Two nickels are impedance matched; the stationary nickel gives as good as it gets, stopping the moving nickel dead. A penny and a nickel are impedance mismatched; a penny does not exert as much force back on the colliding nickel as another nickel would and does not decelerate the nickel all the way to zero speed. The nickel continues on its way, albeit more slowly. Only in the case of equal masses does the energy get completely transferred from one coin to the other; this is clear since for the head-on nickel-penny and penny-nickel collisions, both coins remain in motion and that movement carries energy. — Eric J. Heller, Why You Hear What You Hear- An…, 60

Categorization

• DEFINE: Builds the impedance-matching concept from coin collisions as an intuitive physical foundation.
• XREF: Fundamental principle that later underlies why sound reflects at boundaries between different media — matches the reader’s why-do-we-hear-what-we-hear question.