Two equal masses colliding elastically exchange velocities completely — the moving ball stops, the stationary one takes off at the exact same speed. Any energy lost to heat or sound (an inelastic collision) breaks this clean exchange.
Momentum is always conserved in a collision, but kinetic energy is only conserved in the idealised elastic case — real-world collisions sit somewhere between the two extremes depending on how much energy the materials absorb rather than transfer.
Estimate with the rule, then check it against the calculator that models it properly.
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