SUPERBOUNCES (Oct 2007, Dec 2009)

A popular show isto drop 2 balls together, to speak a tennis ball on top of a basketball. The tennisball climate bounces with about 16 times much more energy, through bouncing off thebasketball, 보다 it does by bouncing directly off the floor. A common, streamlined explanation isthat the basketball bounces very first and then renders a second collision v theincoming tennis ball. In fact, both balls bounce with each other (unless the tennisball is intentionally or coincidentally dropped soon after the basketball). Seefor yourself, in ~ 300 frames/sec, here.

A detailed explanation that the processinvolved is offered in AmericanJournal the, 75, 1009-1016 (2007). If the bigger ball is ~ above top,then the little ball gets trapped between the large ball and also the floor and canbounce countless times before the two balls separate. A movie is presented here, however themultiple bounces can�t be viewed in the movie because they happen too quickly. Theycan be seen much much more easily through bouncing the balls turn off a piezo disk or forceplate, as explained in the AJP paper.

You are watching: Balls out physics

SPRINGBOUNCE (Jan 2008, Nov 2009)

Whena ball bounces, the pressure on theball increases to a maximum as soon as the round compression is a maximum, and thendrops earlier to zero at the finish of the bounce period. The force varies in a sinusoidalmanner. When a spring bounces on its end, the force remains consistent in timewhile a compression wave travels approximately the peak end, reflects, and travels backto the bottom end. Climate the force drops come zero and the spring bounces. A 300frames/sec movie reflecting the compression wave can be seen right here (taken in sunlight through a Casio EX-F1 camerausing 1/4000s exposure). The bounce is likewise shown in the diagram below. Themovie is played ago at 30 fps (in slow-moving motion). Note likewise that the bottom ofthe spring starts falling fine AFTER the top is released!


Aninteresting attribute is that the feather bounces after the wave renders two trips(one up and also one down) follow me the spring. When a steel ball bounces, a compression wave travels up and also downthe ball about 15 times prior to the round bounces. The pressure on the sphere is not constant like it is for aspring due to the fact that it take away a lengthy time because that the bottom that the sphere to compress andthen expand.

Whentwo springs or 2 rods collide, and also if the lengths space different, climate kineticenergy is no conserved since the long rod or the long spring is stillcompressed at the finish of the collision. As soon as two steel balls collide, kineticenergy is conserved even if the balls are of different diameter. The reason isthat wave motion during a collision between two balls dram a neglible role.Almost every the elastic energy in the two balls is stored in the small contactvolume and also very little energy is combination to propagating waves since thecollision is spread out end a lengthy time. The collision bring away a lengthy timebecause the call area is quite tiny and relatively soft compared with therest that the ball. The difference between ball and spring collisions isdescribed in more detail here.

SLINKYDROP (Jan 2008, Nov 2009)

Whena slinky feather is suspended at its optimal end and also then released, will certainly the wholespring autumn vertically as soon as the top end is released? Or will the bottomend fall first? Or will the top end fall first? Think it v then examine your answer below (filmed at300 frames/sec). It�s quitesurprising. Check out Am. J. Phys. P 583 - 587, July 2007 for an explanation.

Asimilar thing happens as soon as a player strikes a sphere with a bat or society orracquet. The impact sends a transverse wave along the implement, yet the ballis fine on its method by the moment the bending wave arrives at the player�s hands.So, anything an intricate the player does through the hands throughout or after ~ striking theball is purely for show. The only duty of the hand after the affect is come bringthe implement come a stop.


Most ballsused in sport space spherical. Some space elongated, favor oval shaped footballs.They space subject come the exact same ground reaction and also friction forces as any kind of otherball, yet the regular reaction force rarely acts v the center of mass ofthe ball. This results in a novel bounce effect whereby elongated balls have tendency tobounce in the direction they space pointing when they fight the ground. To research this effect, i made a fatpencil native a plastic tube v an eraser grounding in one end. The bounce of suchan object have the right to be fairly amusing, as shown in the fastened movie. The bounce the such an item shows clearlythat revolution friction is often an ext important than sliding friction during thebounce. Sliding friction have the right to bring an object to rest in the slide directionbut that can�t turning back the direction that motion and also it can�t accelerate an objectin the slide direction. Onlystatic friction can do that.

The plussign ~ above the pencil clues its centre of mass and the vertical white line in themiddle the the movie is a plumbob. The movie is not really entertaining as soon as viewedin genuine time. You need to breakthrough it one framework at a time to slow-moving it down.

Fallingpencils (or trees or chimneys) can additionally behave in unforeseen ways,as presented in this movie and also as explained in this paper.


Footballstend come bounce in the direction they are pointing when they land, as shown in MovieA. This is not always the case, due to the fact that the bouncedirection also depends top top the spin, the direction that the spin and also the initialforward speed. MovieB reflects the unexpected natureof part football bounces. These two movies were filmed at 25 frames/s yet eachframe was break-up in half. The top fifty percent was taped 10 ms before the bottomhalf, the moment interval between each framework being 40 multiple sclerosis in both halves the themovie. Various other bounces are displayed on the Movie Clips page.

A scientific file on thissubject, including the results of 200 various bounces at miscellaneous angles andspins, can be downloaded as a 700 kb pdf file.


Maximum stress and anxiety in a balloccurs in the small an ar where the sphere contacts the surface. A roughindication of the tension pattern is displayed in the complying with photos of a 1 mmthick sheet of polycarbonate compressed edge-on (top come bottom) and viewedthrough two sheets of crossed polaroid. The polycarbonate sheet was reduced withscissors to have a flat surface at the top and also a curved surface at the bottom.

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The stress is clear concentratedin the contact region but extends roughly the edge of the sheet together thecompression pressure increases as result of bending of the polycarbonate sheet. Thephotos were taken in room light through a sheet of white cardboard at the behind toreflect light through the system. The polycarbonate paper is between two largesheets of polaroid.