Hit a stationary snooker ball dead-on with the cue ball, and something a little strange happens: the cue ball stops almost dead in its tracks, and the ball it hit shoots off at close to the cue ball's original speed. It looks like the cue ball vanished its own motion into the other ball — because, in a very real sense, it did. That handover is what an elastic collision looks like when the two objects have (roughly) equal mass.
Two things have to balance, not just one
Every collision conserves momentum — that's true whether it's elastic or not, no exceptions. What makes a collision specifically elastic is a second, stricter condition: kinetic energy is conserved too, not just converted into heat, sound or permanent deformation. Most real-world collisions — a car crash, a dropped egg, a ball of Blu Tack hitting a wall — conserve momentum but lose a lot of kinetic energy to those other forms. An elastic collision is the special case where none of it leaks away.
Equal masses, dead-on collision: ball A transfers all of its velocity to ball B and stops. Watch the loop — A's motion doesn't disappear, it moves to B.
Why snooker balls get so close to it
Snooker and pool balls are hard, rigid, and barely deform on contact, which is exactly the physical condition an elastic collision needs — energy that would otherwise bend or dent the material instead stays as kinetic energy in the balls' motion. That's not a coincidence of the game's design; it's why the balls are made from a hard resin rather than something softer. A game played with balls of clay or dough would behave completely differently, because most of the energy would vanish into deforming the ball instead of into its speed after impact.
What happens when the masses aren't equal
The clean velocity swap only happens because ball A and ball B have (roughly) the same mass. Hit a heavier, stationary ball with a lighter one, and both keep moving afterwards — the lighter ball bounces back at reduced speed instead of stopping, and the heavier one moves off, but slower than the lighter one was travelling. The two rules from earlier — momentum conserved, kinetic energy conserved — still hold exactly; they just no longer allow a total handover, because a bigger object needs less speed to carry the same momentum, and matching both conditions at once forces this specific split rather than a full swap.
Unequal masses: the lighter ball (m) rebounds instead of stopping, and the heavier ball (2m) moves off more slowly than m arrived — momentum and kinetic energy are still both conserved, exactly.
The collision hiding inside every gas law
Kinetic theory treats gas particles as constantly colliding — with each other and with the walls of their container — and assumes every one of those collisions is perfectly elastic. That single assumption is what makes gas pressure and temperature stable quantities rather than ones that quietly decay over time: if gas particles lost kinetic energy on every collision the way a dropped ball does, a sealed container of gas would slowly cool itself down for no external reason, which isn't what happens. The same idea that explains a snooker shot is quietly running underneath Boyle's law, Charles's law and every other gas law taught at GCSE and A-Level. If momentum, kinetic energy or the gas laws need explaining with the actual mechanism rather than a formula to memorise, that's exactly what our GCSE physics tutoring is for — see the full learning pathway here.
Frequently asked questions
Is a Newton's cradle a perfect elastic collision?
Almost, but not quite — a small amount of energy is always lost to sound (that click) and to slight deformation of the balls, which is exactly why the swing height very gradually decreases if you leave it running. A truly perfect elastic collision, with zero energy loss, is an idealisation that real objects only approximate closely.
Why does the equal-mass case look so different from unequal masses?
With equal masses, the maths simplifies to a clean velocity swap — ball A stops, ball B leaves at A's exact speed. With unequal masses, both balls generally keep moving after the collision, just at different speeds than before, because a single lighter or heavier ball can't fully absorb and re-emit the momentum the way an identical one can. The underlying conservation laws are the same either way; only the outcome looks different.
Do gas particles really collide elastically all the time?
In the kinetic theory model used at GCSE and A-Level, yes — gas particle collisions are treated as perfectly elastic, which is precisely why a gas at constant temperature doesn't gradually lose energy and cool down on its own. If collisions between gas particles weren't elastic, gas pressure and temperature would slowly decay with no external cause, which isn't what's observed.
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Sudershan Soni
Founder & Lead Tutor at Mostak Services — an MSc-qualified Mathematics, Science, Computer Science & STEM tutor with 20+ years of professional experience, teaching students from 11+ and GCSE to A-Level and beyond, online worldwide.
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