Odd-Ball Bounces

Most balls used in sport are spherical. Some are elongated, like oval shaped footballs. They are subject to the same ground reaction and friction forces as any other ball, but the normal reaction force rarely acts through the centre of mass of the ball. This results in a novel bounce effect whereby elongated balls tend to bounce in the direction they are pointing when they hit the ground. To study this effect, I made a fat pencil from a plastic tube with an eraser stuck in one end. The bounce of such an object can be quite amusing, as shown in the attached movie. The bounce of such an object shows clearly that static friction is often more important than sliding friction during the bounce. Sliding friction can bring an object to rest in the sliding direction but it can’t reverse the direction of motion and it can’t accelerate an object in the sliding direction. Only static friction can do that.

The plus sign on the pencil marks its centre of mass and the vertical white line in the middle of the film is a plumbob. The movie is not very entertaining when viewed in real time. You need to advance it one frame at a time to slow it down.

Football Bounce

Footballs tend to bounce in the direction they are pointing when they land, as shown in MovieA. This is not always the case, since the bounce direction also depends on the spin, the direction of the spin and the initial forward speed. MovieB shows the unexpected nature of some football bounces. These two movies were filmed at 25 frames/s but each frame was split in half. The top half was recorded 10 ms before the bottom half, the time interval between each frame being 40 ms in both halves of the movie.

A scientific paper on this subject, including the results of 200 different bounces at various angles and spins, can be downloaded as a 700 kb pdf file. The paper has not yet been published (in any journal).