Enter the two-handed bowler. Increasingly, we are seeing this novel technique cropping up in bowling alleys across the country. Notice the formidable hook you can generate with this type of delivery -- it looks like the ball is headed straight for the gutter, but then, seemingly at the last second, it cuts back into the pocket for another strike. It's this superior hooking ability that makes two-handed bowling a force to be reckoned with. In order to get some insight into the issue, let's examine some of the physics involved in tossing a 12- to 16-pound sphere down a lane of polished oily wood.
In order to get a strike you probably already know that the ball needs to strike the pins in one of the "pockets", which are the regions halfway between the head pin and the pins on either side of the head pin. But why do we need to hook the ball at all? Why not just throw it straight up the alley and directly into the pocket? The answer has to do with conservation of momentum. When the ball impacts the head pin in a glancing collision, it imparts some of its momentum to the pin. Say the ball knocks the head pin forward and to the left. In order for momentum to be conserved, the ball has to rebound (forward) and to the right. If the ball hits the pocket in a straight roll, the rebound tends to knock the ball away from the main concentration of pins, often leaving you with a disconcertingly difficult spare to contend with.
However, if the ball hooks into the pocket at a steep angle, there is less rebound, and the ball finds itself right in the middle of the action. The result: a strike and that uniquely characteristic and aesthetically pleasing sound of bowling pins flying in all directions.
In order to hook a bowling ball, you need to give it spin. Now obviously, simply rolling the ball down the alley will make it end up spinning. However, the spin is around an axis perpendicular to the alley. In order to curve the ball, you need it to also spin around an axis parallel to the alley. You need to give it a side spin.Now, notice how the ball always seems to hook very late in its roll. This is because bowling lanes are oiled, but only partway up the lane. The lane is dry the last 10 feet or so in front of the pins. The result is that, due to the low friction from the oil, a ball will tend to slide until it gets to the drier part of the lane. At that point there is sufficient friction for rolling to commence. Without side spin, the ball just rolls forward, but if you give the ball a side spin -- let's say you're a right-handed bowler so you give the ball a spin to the left -- as the ball spins it pushes against the lane towards the right. According to Newton's third law the lane reacts by pushing back on the ball with an equal amount of force to the left. It's this reaction force that causes the ball to then accelerate in that direction. The faster you can get that ball spinning, the harder it pushes into the lane, and the more hook you're going to get. And so ... enter the two-handed bowler!
Nowadays, they actually make high-performance bowling balls so that they will naturally tend to hook. They do this by incorporating an asymmetrical core into the ball. The old symmetrical plastic balls don't curve nearly as easily, which makes those hooks in the video doubly impressive, if, as they claim, he's using a plastic ball.
Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.
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