SUPERBOUNCES (Oct 2007, Dec 2009)

A famous demonstration isto drop 2 balls together, say a tennis sphere on peak of a basketball. The tennisround then bounces via about 16 times more energy, by bouncing off thebasketsphere, than it does by bouncing straight off the floor. A common, simplified explacountry isthat the basketball bounces first and also then renders a second collision through theincoming tennis round. In fact, both balls bounce together (unmuch less the tennissphere is deliberately or accidentally dropped quickly after the basketball). Seefor yourself, at 300 frames/sec, right here.

A thorough explacountry of the processinvolved is provided in AmericanJournal of, 75, 1009-1016 (2007). If the bigger round is on optimal,then the tiny round gets trapped between the substantial ball and the floor and also canbounce many times prior to the 2 balls separate. A movie is shown here, yet themultiple bounces can�t be checked out in the movie since they occur as well conveniently. Theycan be seen much more easily by bouncing the balls off a piezo disk or forceplate, as defined in the AJP paper.

You are watching: Balls out physics

SPRINGBOUNCE (Jan 2008, Nov 2009)

Whena sphere bounces, the force on theball boosts to a maximum once the ball compression is a maximum, and also thendrops earlier to zero at the finish of the bounce duration. The pressure varies in a sinusoidalmanner. When a spring bounces on its end, the force continues to be continuous in timewhile a compression wave travels approximately the optimal finish, mirrors, and travels backto the bottom finish. Then the pressure drops to zero and also the spring bounces. A 300frames/sec movie mirroring the compression wave have the right to be checked out below (taken in sunlight through a Casio EX-F1 camerautilizing 1/4000s exposure). The bounce is also displayed in the diagram below. Themovie is played ago at 30 fps (in slow-moving motion). Keep in mind also that the bottom ofthe spring starts falling well AFTER the height is released!


Anamazing attribute is that the spring bounces after the wave makes two trips(one up and also one down) along the spring. When a steel ball bounces, a compression wave travels up and downthe ball around 15 times prior to the sphere bounces. The force on the sphere is not consistent like it is for aspring since it takes a lengthy time for the bottom of the round to compress andthen expand also.

When2 springs or 2 rods collide, and if the lengths are various, then kineticpower is not conoffered since the long rod or the long spring is stillcompressed at the end of the collision. When two steel balls collide, kineticpower is conoffered even if the balls are of various diameter. The factor isthat wave activity during a collision in between two balls plays a neglible function.Althe majority of all the elastic power in the two balls is stored in the small contactvolume and also extremely bit energy is coupresulted in propagating waves since thecollision is spcheck out out over a long time. The collision takes a long timebereason the contact location is quite little and fairly soft compared with theremainder of the sphere. The difference between round and spring collisions isexplained in even more detail here.

SLINKYDROP (Jan 2008, Nov 2009)

Whena slinky spring is suspended at its top end and also then released, will certainly the wholespring fall vertically as quickly as the height finish is released? Or will certainly the bottomfinish loss first? Or will the peak end loss first? Think it through then check your answer below (filmed at300 frames/sec). It�s quitesurprising. See Am. J. Phys. p 583 - 587, July 2007 for an explacountry.

Acomparable point happens once a player strikes a round through a bat or club orracquet. The influence sends a transverse wave along the implement, but the ballis well on its method by the time the bfinishing wave arrives at the player�s hands.So, anypoint fancy the player does through the hands in the time of or after striking theball is pucount for present. The only duty of the hands after the impact is to bringthe implement to a sheight.


Many ballsoffered in sport are spherical. Some are elongated, favor oval shaped footballs.They are subject to the exact same ground reaction and friction forces as any kind of otherball, but the normal reaction force seldom acts via the centre of mass ofthe ball. This results in a novel bounce impact whereby elongated balls tend tobounce in the direction they are pointing when they hit the ground. To study this result, I made a fatpencil from a plastic tube via an eraser stuck in one end. The bounce of suchan item can be quite amusing, as displayed in the attached movie. The bounce of such a things mirrors clearlythat static friction is frequently even more vital than sliding friction throughout thebounce. Sliding friction deserve to carry a things to remainder in the sliding directionhowever it can�t reverse the direction of motion and also it can�t acceleprice an objectin the sliding direction. Onlystatic friction can execute that.

The plussign on the pencil marks its centre of mass and the vertical white line in themiddle of the film is a plumbob. The movie is not incredibly entertaining as soon as viewedin real time. You should advancement it one frame at a time to slow it dvery own.

Fallingpencils (or trees or chimneys) have the right to also behave in unintended ways,as shown in this movie and as described in this paper.


Footballstend to bounce in the direction they are pointing when they land also, as displayed in MovieA. This is not constantly the instance, because the bouncedirection likewise counts on the spin, the direction of the spin and the initialforward speed. MovieB reflects the unexpected natureof some footsphere bounces. These two movies were filmed at 25 frames/s however eachstructure was split in fifty percent. The optimal fifty percent was taped 10 ms prior to the bottomhalf, the time interval in between each frame being 40 ms in both halves of themovie. Other bounces are presented on the Movie Clips web page.

A clinical paper on thissubject, including the outcomes of 200 various bounces at miscellaneous angles andspins, deserve to be downloaded as a 700 kb pdf file.


Maximum tension in a balloccurs in the tiny area wbelow the ball contacts the surchallenge. A roughindication of the stress and anxiety pattern is displayed in the following photos of a 1 mmthick sheet of polycarbonate compressed edge-on (top to bottom) and also viewedthrough two sheets of crossed polaroid. The polycarbonate sheet was cut withscissors to have a level surchallenge at the optimal and also a curved surface at the bottom.

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The tension is obviously concentratedin the contact area yet extends approximately the edge of the sheet as thecompression pressure rises as a result of bending of the polycarbonate sheet. Thephotos were taken in room light via a sheet of white cardboard at the rear toreflect light through the device. The polycarbonate sheet is in between 2 largesheets of polaroid.