Lift one ball on a Newton’s cradle and release it. The middle balls appear almost still, yet one ball at the opposite end swings away. The incoming ball has not crossed the row or touched the last ball. Brief compression at the contacts carries force through the chain, while conservation of momentum and near-conservation of kinetic energy strongly constrain the outgoing motion.
The short answer
For equal masses in an almost elastic, one-dimensional collision, the simple result that conserves both momentum and kinetic energy is for the incoming ball to stop and one ball at the far end to leave at nearly the same speed.
Observation and interpretation
| Visible in the clip | Interpretation from sources | Not measured |
|---|---|---|
| One end ball strikes the touching row. | The contact collision transfers momentum and energy into the chain. | The exact incoming speed. |
| The middle ball centers move very little. | Small elastic deformation and force can propagate faster than the centers visibly shift. | Contact compression and pulse travel time. |
| One far-end ball swings out. | Equal masses and a nearly elastic collision favor a one-in, one-out state. | The fraction lost to sound, heat and vibration. |
Step 1: the lifted ball converts height into motion
Raising the first ball gives it gravitational potential energy. After release, that energy becomes motion, and the ball reaches its greatest speed near the bottom of the arc. At impact, its momentum and kinetic energy enter the system made by all the balls.
Harvard describes Newton’s cradle as a demonstration of elastic collisions between metal balls and of conservation of momentum and energy. Its demonstration notes that moving one ball sends one from the far end, while moving two or three sends the same number from the other side.
Step 2: momentum alone does not select one outgoing ball
Suppose one ball of mass m arrives with speed v. If one equal ball leaves with speed v, momentum is mv before and after. Kinetic energy is also the same, one-half mv squared.
Momentum by itself allows other imagined distributions. Two outgoing balls each moving at v/2 would still have total momentum mv. Their combined kinetic energy, however, would be only half of the original value. Adding the near-elastic condition rules that state out as a good description of the cradle. OpenStax states that momentum is conserved in a closed collision system and that kinetic energy is also conserved in an elastic collision.
The two conservation laws therefore explain the count. One equal ball in corresponds to one equal ball out at about the same speed. If two balls arrive together at the same speed, two outgoing balls are the matching ideal result.
Step 3: the middle balls transmit a contact pulse
Steel looks rigid, but each contact compresses by a tiny amount during impact and then recovers. The first contact loads the next ball, successive contacts respond, and the final ball receives an outward force as the chain relaxes. A middle ball can participate in this process while its center moves too little and too briefly to notice in an ordinary clip.
The University of Maryland explains that the hard steel spheres collide nearly elastically, so most incoming momentum and energy reaches the outgoing sphere. Maryland also notes that a more detailed model can treat the balls as coupled oscillators with a finite propagation speed or as slightly inelastic masses.
Why do all the balls not drift together?
If every ball left together at a much lower common speed, total momentum could be shared among them, but much of the original kinetic energy would be missing. A real cradle does lose some energy, but hard steel contacts over a short collision are elastic enough that the energy-conserving constraint remains a useful approximation. That is why the motion looks different from a sticky collision in which objects move away as one group.
Testing another explanation
Calling the row a perfectly rigid rod that instantly pushes the last ball is incomplete. It skips the finite sequence of compression and recovery at the contacts; the middle centers can barely move while force and elastic deformation still pass through them.
The conservation laws restrict the state before and after impact. Elastic contact explains how the row produces that result during the short collision. Both levels are needed: one gives the allowed outcome, and the other gives the transmission process.
A prediction you can check
With equal balls touching and aligned, releasing one ball from a fixed height should send roughly one ball from the far end to a similar height; releasing two together should send roughly two from the far end.
Film the cradle in slow motion and repeat one-ball and two-ball releases from the same marked height. Compare the outgoing count and maximum height. Then add a soft piece of tape to one contact or introduce a small gap; the larger loss and altered contact sequence should make the pattern less clean.
Rewatching the opening shot
The quiet-looking middle balls are not spectators. Their contacts compress and recover while transferring force through the row. The first ball does not travel through the chain; its incoming momentum and energy reappear mainly as motion of the far ball. The familiar one-in, one-out pattern is the visible result of equal masses, alignment and an impact that is close to elastic.
Sources checked
Sources & further reading
- sciencedemonstrations.fas.harvard.edu — Newtons cradle
- lecdem.physics.umd.edu — 541 demo highlight newton s cradle blog.html
- openstax.org — 9 4 types of collisions
References checked 2026-09-16. This is an adapted English edition, not an endorsement by the cited organizations.
Watch the original scene · English-language clip
Open on YouTube ↗Something needs correcting? Contact the editorial desk with the article URL and supporting evidence.
