Kinetica's wall is perfectly bouncy: the disc leaves exactly as fast as it arrived, losing not a scrap of speed. A collision that keeps all its kinetic energy like that is called elastic — and real-world bounces only ever come close.
Watch the speed readout: it never falls. Kinetica's wall is perfectly elastic, so the disc rebounds at exactly the speed it struck, keeping all its kinetic energy. A real ball would lose a little each bounce and finally stop — here it never does.
An elastic collision is one in which the total kinetic energy is conserved — none is lost to heat, sound, or deformation. Bouncing off an immovable wall, the object leaves with exactly the speed it arrived. The "bounciness" is captured by the coefficient of restitution, e = speed out ÷ speed in. Perfectly elastic means e = 1.
In every collision momentum is conserved. In an elastic collision kinetic energy is conserved as well. Real collisions are inelastic: some kinetic energy becomes heat, sound, and permanent dents, so the object rebounds slower, with e < 1. A perfectly elastic collision (e = 1) is an idealisation — hard steel balls and gas molecules come close, but nothing is truly perfect.
The Table's wall is perfectly elastic, e = 1. The disc rebounds at exactly the speed it struck, so its kinetic energy never drains away and it bounces forever. A real disc would lose a little speed at each bounce and eventually stop; Kinetica's idealised wall lets the motion continue without end.
Bounciness becomes concrete when you dial the restitution and watch a dropped ball either climb back to the same height or fade away, bounce by bounce.
Each bounce reaches e² of the previous height, because kinetic energy goes as speed squared. At e = 1 (Kinetica's wall) the ball never loses height.
Every real ball bounces back a little lower, because the collision is slightly inelastic — some energy escapes as heat and a thud of sound. A lively basketball has a restitution around 0.8; a beanbag, near zero, barely bounces at all.
The polished steel balls collide almost elastically, passing motion cleanly from one end to the other with very little loss. It keeps clicking for a long time precisely because the collisions waste so little kinetic energy.
A crumple zone is deliberately inelastic. By folding and absorbing kinetic energy in a crash, it lengthens the stop and softens the forces on the passengers — here, losing energy in the collision is exactly the goal.
An elastic collision is the idealised bounce that loses no energy — the limit real collisions only approach. This FAQ separates elastic from inelastic, introduces the coefficient of restitution, and follows it into bouncing balls, Newton's cradles, and crumple zones.
An elastic collision is a bounce in which no kinetic energy is lost. The object leaves with exactly the speed it arrived, so nothing is wasted as heat, sound, or a dent. Off a fixed wall, a perfectly elastic bounce sends an object back just as fast as it came in. Kinetica's wall behaves this way, which is why its disc can bounce forever without slowing down.
| Property | Elastic | Inelastic |
|---|---|---|
| Kinetic energy | Conserved | Partly lost |
| Restitution e | 1 | Less than 1 |
| Rebound speed | Same as impact | Slower |
| Momentum | Conserved | Conserved |
The coefficient of restitution, written e, is a single number that measures how bouncy a collision is. It is the speed of separation divided by the speed of approach — roughly, speed out divided by speed in. It runs from 0 to 1: e = 1 is a perfectly elastic bounce that keeps all the speed, while e = 0 is perfectly inelastic, with the objects sticking together. A lively ball might have e around 0.8.
Yes. Momentum — mass times velocity — is conserved in every collision, elastic or inelastic, because the colliding objects push on each other with equal and opposite forces. What changes between the two types is kinetic energy: an elastic collision keeps it all, while an inelastic one converts some into heat, sound, and deformation. So momentum conservation is universal; kinetic-energy conservation is the special elastic case.
The missing kinetic energy is not destroyed — it is converted into other forms. Most becomes heat as the materials flex and rub internally, some radiates away as the sound of the impact, and some goes into permanently bending or denting the objects. Add all these up and the total energy is still conserved; it has simply left the organised motion of the objects and spread into less useful forms.
Because each bounce is slightly inelastic, so the ball leaves the floor a touch slower than it landed. Since the height a ball reaches depends on its speed, a small loss of speed means a noticeably lower bounce. The fraction of height kept each time is e², so a ball with restitution 0.8 returns to about 64% of its previous height, then 64% of that, fading away over several bounces.
A restitution of e = 1 means the collision is perfectly elastic: the object separates exactly as fast as it approached, losing no kinetic energy at all. A ball with e = 1 dropped onto a floor would bounce back to precisely the same height, again and again, forever. It is an idealisation — real materials always have e a little below 1 — but it is exactly how Kinetica's frictionless, perfectly bouncy wall behaves.
A perfectly inelastic collision is one with e = 0, in which the colliding objects stick together and move off as one. It loses the maximum possible kinetic energy while still conserving momentum. A lump of clay hitting the floor and stopping dead, or two railway carriages coupling on impact, are everyday examples. The objects do not bounce apart at all, which is the opposite extreme from a perfectly elastic collision.
Very nearly. The collisions between gas molecules are treated as almost perfectly elastic, because the molecules bounce off one another with virtually no kinetic energy lost. This is a key assumption behind the kinetic theory of gases, which explains pressure and temperature from countless tiny collisions. If those collisions were strongly inelastic, a sealed gas would gradually lose its motion and cool on its own, which it does not.
A Newton's cradle uses hard, polished steel balls, whose collisions lose very little kinetic energy and so are nearly elastic. When one ball swings in, the momentum and energy pass cleanly along the line and pop a ball off the far end at almost the same speed. Because so little energy is wasted as heat and sound, the cradle keeps clicking back and forth for a long time before it finally slows and stops.
A crumple zone is built to crush in a crash, deliberately turning the car's kinetic energy into the work of bending metal rather than letting it snap back. This inelastic collapse lengthens the time and distance over which the car stops, which lowers the forces on the passengers. A perfectly elastic car would rebound violently and pass the full shock straight to the occupants, so here losing energy in the collision is a life-saving feature.
The coefficient e is a ratio of speeds, so after a bounce the ball moves at e times its landing speed. But the height a ball reaches depends on its kinetic energy, which goes as speed squared. Squaring the speed ratio gives the energy and height ratio, so the rebound height is e² times the drop height. That is why a modest speed loss, say e = 0.9, still drops the height to 81%, and the effect compounds with each bounce.
Not from the collision itself. With no added energy, a bounce can at best keep all its kinetic energy, so e cannot exceed 1. A value above 1 would mean the object leaves faster than it arrived, which only happens if a stored or chemical energy source pushes during contact — a spring-loaded bumper, an explosion, or a struck superball trick. For ordinary passive collisions, e ranges from 0 to 1.
Engineers tune restitution for the job. Sports bodies set allowed bounciness for balls and bats so games stay fair and predictable. Designers of packaging, helmets, and crumple zones make collisions inelastic to absorb shocks and protect what is inside. Makers of trampolines, springs, and bouncy toys do the opposite, maximising restitution. Knowing how elastic a collision will be is central to designing how things bounce, or refuse to.
Everywhere things hit each other. A dribbled basketball, a struck pool ball, and a Newton's cradle are nearly elastic; a dropped phone, a car crash, and a caught cricket ball are decidedly inelastic. Sports, road safety, packaging, and even the behaviour of gases all hinge on how much kinetic energy a collision keeps or loses. The coefficient of restitution quietly sets how bouncy our world is.
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