Elastic collisions conserve both momentum and energy — real ones rarely do

What is the difference between elastic and inelastic collisions?

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.

Why it works

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.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

Open Collision Simulator →

More in Physics

• Hohmann is the cheapest transfer — until the ratio passes about 12Two burns on opposite sides of an ellipse is the minimum-energy route between circular orbits, a…
• A narrower gap spreads light more, not lessDiffraction angle goes as wavelength over aperture, so halving the opening doubles the spread — …
• Geostationary runs on the sidereal day, not the 24-hour oneThe altitude falls straight out of the period: match Earth’s rotation and the orbit radius is fi…
• The peak wavelength is about 2,900 divided by the temperatureA hot object shines brightest at a wavelength, in micrometres, of about 2,900 divided by its tem…

Browse the full Rulebook →