A cricket ball, a shuttle, and a rugby ball walk into a physics video...

Hi everyone, for this video I decided I would combine my interest in sports with my passion for physics, and discuss the science behind the 3 most interesting sports balls I can think of. Technically, we'll be looking at projectiles used in sports, rather than balls, since they aren't exactly spherical.

The first ball we will be looking at is the cricket ball. This ball is made up of a cork interior, wrapped in string, and then covered with two hemispheres of leather. The leather half-spheres are then stitched together to create the seam. The seam is what is responsible for lots of the interesting behaviour we see from a cricket ball.

For example, in spin bowling, the bowler will impart rotations on the ball as well as throwing it from one end of the pitch to the other. These rotations will cause the ball to exert a force on the pitch when it lands. Due to Newton's Third Law of Motion ("every action has an equal and opposite reaction"... eugh) the pitch will exert a force on the ball in the opposite direction, and hence the ball's direction will change as soon as it bounces. This change in direction is primarily what a spin bowler relies on to deceive the batter at the other end.

Another type of bowling used in cricket is swing bowling. This happens when the ball moves laterally (sideways) through the air, not just when it pitches on the ground. This lateral movement is caused by the air molecules exerting an uneven force on the ball as it moves through the air. This happens because the seam is angled in a particular direction by the bowler. One half of the ball presents a smooth surface to the air particles flowing past, which means they can flow in nice smooth sheets - laminar flow. The other half of the ball is where the seam presents itself to the airflow. The seam is rough, so only allows turbulent air to flow over the surface of the ball. The turbulent air flow separates later from the ball than the laminar flow on the other side, so the ball essentially pushes the air off to one direction behind itself. This means the air exerts an uneven force on the ball (by Newton's Third Law again) and the ball deviates through the air.

Moving on to the next sport, we'll also be considering badminton in this video. The projectile used in badminton is the shuttlecock / shuttle / birdie / ball. It consists of a hemispherical cork base, with 16 goose feathers inserted into it in a very specific arrangement.

The shuttle has a very high drag coefficient, so its trajectory when hit high and far is not like a parabola, which is what we commonly see with projectiles largely unaffected by air resistance. The shuttle drops a lot more sharply towards the end of its trajectory, as it loses a lot of speed.

Still, badminton is the fastest racket sport in the world - with shots regularly being recorded as being played over 400 km/h. Aside from this, the specific arrangement of feathers in the cork base means that even if left handed players and right handed players play the exact same shot on the same shuttle (but mirrored of course), the shuttle will not respond identically (but mirrored). The arrangement of feathers means that a left handed player's slice shot will result in a slightly different trajectory to a right handed player's slice shot.

Finally, the last sport with a weirdly shaped ball that we will study is rugby. The rugby ball looks similar to an American football, but aren't quite identically shape. The fact that the rugby ball is less pointy means that it's much more prone to tumbling end on end.

How rugby players avoid this is to impart rotation on the rugby ball, which makes its motion much more smooth. This is due to the Law of Conservation of Angular Momentum. The rotation gives the ball angular momentum, meaning the air that it interacts with on its path from point A to point B largely cannot do enough to overcome the angular momentum and cause the ball to move unpredictably. Whereas if the ball is not given angular momentum as it is thrown, then the air particles definitely have enough force / torque to cause the ball to rotate along any random axis, increasing the likelihood of an inaccurate tumbling throw.

Thumbnail photo credit: Siora Photography

Here are the clips referenced in my video:
   • King Kohli does it again in Adelaide  
   • Full Countdown: Shane Warne's 50 best...  
   • James Anderson at his best, showing h...  
   • Zheng/Huang vs Watanabe/Higashino sem...  
   • 20 Fastest SMASHES in Badminton  
   • Video  
   • 3 Types of Kevin Sanjaya SERVE  
   • Learn The MAGIC Slice Shot in Badminton  
   • slow motion rugby ball pass  

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