Several tennis balls are already bouncing in the opening frame. Follow one ball and its arc becomes smaller after each contact with the court. During impact, the ball and surface deform and briefly store part of the incoming motion. Only part of that energy returns as upward motion. The rest spreads into internal friction, heat, sound, vibration and deformation of the court.
The short answer
A real ball-floor collision is not perfectly elastic. Each impact returns less upward speed than the ball had downward speed, so the ball begins the next rise with less kinetic energy and reaches a lower peak.
What the clip shows, and what it does not measure
| Visible in the clip | Interpretation from sources | Not measured |
|---|---|---|
| Tennis balls strike the court and rise again. | The ball and surface deform, store elastic energy briefly, and return some of it. | Contact time and maximum compression. |
| Successive arcs reach lower peaks. | The ratio of rebound speed to impact speed is below one. | The coefficient of restitution for any one impact. |
| Different balls appear at different heights. | The scene demonstrates repeated rebound, not a controlled comparison among balls. | Each ball’s pressure, spin and exact drop height. |
Step 1: falling converts height into speed
A raised ball has gravitational potential energy. As it falls, its height decreases and its speed increases. If air resistance is small over the short drop, most of that large-scale energy is kinetic just before the court stops the downward motion. The faster the approach, the more the ball and court must deform during their brief contact.
Gravity also slows the ball on the way back up, but that alone would not explain why it fails to regain its previous height. The decisive comparison is the speed immediately before impact with the speed immediately after impact.
Step 2: the impact temporarily stores energy in deformation
The bottom of the ball stops first while its center of mass is still moving down. The rubber shell, pressurized interior, felt and court respond as a coupled spring. Compression converts some incoming kinetic energy into elastic deformation. As the ball recovers its shape, that stored energy pushes it upward.
The University of Illinois Physics Van explains that deformation stores energy in the ball, while the portion not returned to large-scale motion becomes heat, sound and other small motions. Rubber flexes with internal friction, fibers shift, the court moves slightly and the audible thump carries energy away from the ball’s upward motion. Energy has not vanished, but it has dispersed into forms that do not reassemble into one rising ball.
Step 3: a smaller rebound speed creates a lower next peak
OpenStax defines the coefficient of restitution for a ball-floor collision as the speed after impact divided by the speed before impact, and relates it to the square root of rebound height divided by drop height. A value of one describes an ideal perfectly elastic rebound. A real basketball or tennis ball on a real surface has a value below one under ordinary conditions.
Because kinetic energy depends on speed squared, even a modest reduction in rebound speed makes a visible difference in peak height. The ball then falls from that lower peak, arrives at the next impact with less large-scale energy, and loses another portion. If conditions remain similar, the sequence looks like a shrinking staircase of arcs. This clip has no scale, so it does not justify claiming that every height shrinks by exactly the same fraction.
A ball is not described by one permanent bounce number
Rod Cross measured force waveforms, contact time, compression and coefficient of restitution for several balls, and reported that a tennis ball’s restitution changes with impact speed. The measured response includes stages of felt and rubber compression rather than a perfectly simple spring. A coefficient can summarize one collision condition, but it should not be treated as a universal constant for every speed, ball and surface.
The International Tennis Federation’s 2026 technical booklet measures rebound and deformation under controlled conditions and compares a court surface’s rebound with a rigid reference surface. That procedure controls temperature, conditioning and test geometry because rebound belongs to the ball-surface pair. The tennis balls in this English clip and the basketball in the Korean clip show the same qualitative decay, yet their numerical rebound values cannot be exchanged.
Why the gravity explanation falls short
Saying that each bounce is lower because gravity gets stronger is inadequate. Gravity is essentially unchanged over these small height differences; the changed condition is the ball’s smaller upward speed after a non-perfectly-elastic impact.
The phrase “energy loss” also needs care. Mechanical energy available to lift the ball is reduced, but total energy is conserved. Heat, sound and microscopic vibration remain in the ball, court and surroundings after the visible bounce has faded.
A prediction you can test
Dropping the same ball from the same height should usually produce a lower first rebound on a thick deformable mat than on a hard court, while a substantially underinflated ball should generally rebound lower on the same surface.
Keep the ball, release height, temperature and spin as consistent as practical. Record from the side with a safe height marker and compare the first rebound peak. The prediction concerns relative height, not a guessed coefficient. There is no need to drop a ball near glass, people or valuable equipment.
Rewatching the first scene
The balls are not repeating the same launch after every contact. Each impact resets the next arc with a slightly smaller upward speed. The brief squash and recovery act like an imperfect spring, and the lower peaks are the visible record of mechanical energy spreading into heat, sound, vibration and surface deformation.
Sources checked
Sources & further reading
- openstax.org — 8 5 inelastic collisions in one dimension
- van.physics.illinois.edu — 101
- onlinelibrary.wiley.com — J.1460 2687.1999.00019.x
- www.itftennis.com — 2026 technical booklet.pdf
References checked 2026-09-14. This is an adapted English edition, not an endorsement by the cited organizations.
Watch the original scene · English-language clip
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