Hockey is a game of close and intense interaction, in which players are forced ro turn constantly. With a dozen aggressive skaters on a rink measuring only 200 feet long and 85 feet wide, changing direction is unavoidable-it's part of what makes hockey exciting. Who can forget the quick turns and wiggles of the young Wayne Gretzky as he bafRed his opponents on his way ro the net?
The question is, How do we turn in the most energy-efficient way? Saving energy and momentum is important if you are going ro last through the game. After all, in a closely fought battle such as Game Four of the Flyers-Penguins Stanley Cup semifinals in May of 2000, which lasted into a sixth overtime period, it helps ro have expended as little energy as possible during regulation time. The best strategy to change direction, it so happens, is to move in a circle with a short radius.
The mechanics of circular motion at constant speed is a staple of introductory physics courses. It is relevant not only to sports but to everything that turns, from wheels to propellers to planets. Circu- lar motion is simply described in terms of an acceleration directed toward the center of the circle (hence its name, centripetal) resulting from the continual change in the direction of the velocity. According to Newton's first law, which states that an object will stay at rest or
continue moving in a straight line unless a force is applied to it, there must be a force involved. When you spin around while holding a bucket of water, your hand provides the centripetal force needed to keep the bucket in a circular path. When you let go, the force van- ishes and the bucket momentarily goes in a straight line. Likewise, a hockey player turning a corner must rely on a force to keep to a circular path; this centripetal force comes from his skates gripping to the ice.
The inward acceleration of a body moving in a circle is given by
where R is the radius of the circular path and v is the velocity. There-fore, the tighter the radius, the greater the acceleration and the larger the force needs to be. Just as with linear acceleration, a skater needs to shift his or her center of gravity and lean inward and beside the point of contact on the ice. The faster the speed, the smaller the angle of
this lean (see Fig. 2.7). The angle of the body axis relative to the ice is calculated by combining Equations.
To counteract friction or to increase their speed, hockey players can also accelerate by skating while turning. After all, speed skaters such as five-time Olympic medal winner Bonnie Blair don't simply glide around the curves; they skate hard around them. When Blair turns, her skates are oriented at a slight angle relative to the circular path and her legs alternate at pushing sideways. The net result is both a centripetal force and a forward force, which combine to generate a higher speed through the curve.
There are many ways to slow down-people who are just learning how to skate find that our quickly! But hockey players usually brake simply by straightening up their bodies and turning one or rwo skates sideways, rather like a skier makes a parallel turn . This lateral motion of the blade delivers the greatest resistive force and scrapes a thin layer of ice from the surface. (That I know from experience-some players try to annoy a goaltender by peppering his face with a spray of ice after he's made a save.) Equation 2.3, the formula for obtaining the leaning angle of a skater's body, is valid irrespective of the value of the acceleration, which is negative for a skater who is braking.
The deceleration is determined by the friction force on the blade and therefore depends on, among other factors, how deep the blade penetrates the ice. A sharper blade will dig deeper and allow a shorter stop. Professional hockey players know this; they have their skates sharpened before every game and sometimes between periods.
As professional players approach the end of their careers, sometimes at the age of 40 or older, they gradually lose their skating edge and tend to rely more on their experience to stay competitive. Certainly, when Gordie Howe joined the Hartford Whalers at the age of 51, he didn't have the legs he'd had when he started some 30 years before.
Even so, he was able to compete in the NHL thanks to his experience and deep understanding of the game. Younger hockey players
may be quicker, but they tend to be more vulnerable and crack when under too much pressure, a danger during the NHL playoffs. Because of this, coaches generally prefer to use their more experienced goaltenders in the postseason.
In the winter of 2000, I saw firsthand how experienced players could dominate the game of hockey. A team of NHL old-timers
showed up in my hometown of Moncton, Canada, to take on a local team of younger players from the local Royal Canadian Mounted Police. The NHL team consisted of such hockey legends as Guy Lafleur and Marcel Dionne, formerly of the Montreal Canadiens and the Los Angeles Kings, respectively. Most on the roster were in their 50s, and many of them were probably grandfathers already. It was interesting to see the hockey veterans playing so much better than their opponents, many of them twenty-five years younger. Even though the younger players appeared to be quicker, their speed was to no avail.
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