But despite their importance, they are underfunded and underappreciated. The National Science Foundation is able to allocate only a few million dollars per year to support all of them; much more is needed if society is to have evidence at hand when creating policy to navigate our future. If I were able to attend the march, I'd have binoculars dangling from my neck and a butterfly net in hand, and I would stump for more support for these studies. Instead, while my friends and colleagues are marching, I'll be learning which of my marmot friends survived this year's very heavy snowfall.diw.agencyhype.com/15868-nissan-cefiro.php
I'll shed a tear for some and cheer the survival of others. I'll miss the collegiality of the march, but I will sleep well knowing that I am doing my part to train the next generation of critical thinkers and collecting data that may help us learn about the limits that plants and animals face in adapting to a changing environment. And, once my fingers have warmed, I'll write to my friends and ask them how it went.
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Please log in to add an alert for this article. Science 14 September Vol , Issue Feature Piercing the haze. Human Physiology Smarter, stronger, longer. Working Life Learning to lead. In addition to having an intuitive understanding gained from practice, understanding some of the physics theory behind skiing can help make sense of the mechanics involved, which is particularly useful for those who are just starting out. The skiing physics behind different types of skiing will be discussed. It involves high speed and quick turns down a sloped terrain.
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The skier gains speed by converting gravitational potential energy into kinetic energy of motion. So the more a skier descends down a hill, the faster he goes. The picture below shows a downhill skier. An analysis of the physics is very useful in understanding how this is done. A skier maximizes his speed by minimizing resistance to motion, both from air resistance and snow resistance. A skier minimizes his air resistance drag by reducing his projected frontal area. He does this by going into a crouch position, which along with improving his ability to hold balance results in a lower drag force , which acts in a direction opposite his velocity, slowing him down.
The picture below shows a downhill skier in a crouch position. C is the drag coefficient, which can vary along with the speed of the skier. But typical values range from 0. Body posture and the choice of race suit are two critical areas of focus in the effort to minimize drag. The mass of the skier can also help counteract the drag force F D.
A greater mass means a greater force of gravity acting on the skier, which means that the relative magnitude of F D becomes smaller. As a result, the skier reaches a greater velocity v. In reality though, a heavier skier will generally be "bigger", meaning his projected frontal area will be greater, which may negate some of the gain achieved by being heavier. The net effect, however, is that a heavier skier usually goes faster.
But this may compromise the ability of the skier to quickly go around turns, due to his greater bulk. As mentioned, snow resistance is the other factor directly affecting speed.
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Just like for air resistance, we wish to minimize snow resistance in order to maximize speed. However, the techniques used for minimizing snow resistance are subtle, especially when the skier makes a turn. Amateur less experienced skiers typically skid around their turns. This occurs when the skis are tilted on their edge and the exposed base of the skis "plow" into the snow head on. Although the skidding can be controlled and the turn successfully executed, it ultimately results in a significant loss in speed. This occurs because the "plowing" action generates frictional resistance with the snow, by physically pushing it.
This frictional resistance is significantly more than the resistance seen if the skis were to glide on the snow, either with the base of the skis flat on the snow while moving in a straight line in the direction of the skis , or with the edge of the skis planted into the snow while carving around a turn, to be discussed in the next section.
In both these cases, the skis are pointed in the same direction as their velocity which is the same as the velocity of the skier. This is a necessary requirement for minimizing snow resistance.
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The figure below illustrates a turn that is executed while skidding. The path "swept" by the skis is the result of plowing the base of the skis through the snow. This happens when the skier turns his skis too sharply into the turn. This results in the momentum of the skier not changing direction quickly enough to match the direction the skis are pointed. However, in some cases, a degree of skidding is unavoidable, but the key is to minimize it in order to minimize the speed reduction during the turn. The most efficient turn occurs when the skier does a purely carved turn, in which the ski is pointing in the same direction as its velocity.
The figure below illustrates this. As shown in the figure above, in a purely carved turn there is no skidding, and the only snow resistance present is the very small sliding friction between ski and snow. As a result of this minimal level of friction between ski and snow, the speed reduction of the skier is minimized, and he is able to navigate the course faster. Carving turns is one of the more interesting areas of study in the physics of skiing. It takes quite a bit of skill to execute a carved turn.
World Cup level skiers are typically the only ones who are able to do it consistently. The physics of carved turns will be described next. Carved Turns Ski shape is an important factor. Skis are often manufactured with a sidecut on both sides. The amount of sidecut determines the curvature of the ski, which is of a certain constant radius along the sidecut edge. The figure below illustrates a sidecut ski, where R SC is the sidecut radius. Dbenbenn The sidecut helps skiers make purely carved turns.
It affects the physics by influencing the radius of purely carved turns, as will be discussed. A purely carved turn can be done with a ski that is flat on the snow or tilted at an angle to the snow. This is discussed in more detail in the section on how to prevent ski slippage. When the ski is flat on the snow, a purely carved turn is executed when the radius of the turn R T equals the sidecut radius R SC. This allows the ski to go around the turn without any skidding, since the ski is always pointed in the same direction as its velocity v.
The figure below illustrates a purely carved turn for a ski that is flat on the snow. Reverse camber occurs when the force of the skiers foot on the ski causes the ski to bend in a shallow "U" shape, as shown below. Skis can be manufactured with a camber which is opposite to that shown in the figure above.
In other words, the ends of the ski touch the ground while the middle of the ski is elevated. This is done to control how much the ski flattens out when the weight of the skier is applied to the ski. A flat ski distributes the weight of the skier more evenly over the snow surface, which means the ski doesn't dig into the snow as much, and snow resistance is reduced. This is useful when skiing with no tilt on the skis.
Alternatively, if the skis are manufactured flat, then the middle of the ski would sink more than the ends when the skier's weight is applied, and movement through the snow would be more difficult. However, the amount that the middle of the ski bends when a given weight is applied depends on the stiffness of the ski, which can vary in different skis.
This allows the ski to match the radius of the turn. This tells us that skis must largely be selected based on the skier's weight, and how much reverse camber is desired, which depends on the type of skiing to be done. The projection of the sidecut radius R SC onto the snow surface must be a circle in order for the inside edge of the tilted ski to make a purely carved turn. But since the projection of the sidecut radius onto the snow surface is an ellipse, a purely carved turn is not possible. In other words, the inside edge of the ski would not be able to follow the curve defined by the ellipse.
There would be some skidding as a result of the ski pointing in a direction different from the direction of its velocity v.
Therefore, the ski needs to have a reverse camber such that the sidecut radius, when projected onto the snow surface, is a circle. The reverse camber must be great enough to shorten the length of the semi-major axis so that it equals the length of the semi-minor axis, which gives us a circle or very close to it.
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This allows the inside edge of the ski to make a purely carved turn. This is the radius of the turn for pure carving to occur. In the next section we will take a closer look at carving and how it relates to sidecut edge penetration into the snow. To understand this consider the following. This gap is greater for a smaller sidecut radius, and this gap is smaller for a larger sidecut radius. The maximum amount of reverse camber occurs when this gap is closed — in other words, when the sidecut edge presses into the snow. So the larger the gap, the more reverse camber is possible, since the ski can bend more in the middle before the sidecut edge presses into the snow.
A ski with a smaller sidecut radius and larger gap between sidecut edge and snow surface , can accommodate a greater amount of reverse camber, which means it can carve smaller radius turns. A ski with a larger sidecut radius and smaller gap between sidecut edge and snow surface , can accommodate a lesser amount of reverse camber, which means it is best suited for carving larger radius turns.
Given the complexity of all these inter-related factors, the ability of a skier to make a purely carved turn comes down to his ability to recognize the terrain and make adjustments, based on the factors just mentioned. Clearly, carving adds substantial complexity to the physics involved. In the next section we will look at skidding in more detail. Skidding The physics of skidding is different from the physics of carving. Unlike carving where a skier eases into the turn, a skidded turn is initiated by the skier simultaneously tilting the edge of the skis into the snow and pivoting his skis in the direction he wants to turn.
The Physics of . . . Skiing
This results in the skier turning in that direction, due to the plowing effect of the skis with the snow. The illustration below shows the beginning of a turn, in which the skis are initially pointed in a certain direction, at a certain steering angle. Since the skis are pointed in a direction that is different from the initial velocity v i , it is unavoidable that skidding will occur. The steering angle determines how much the skier turns, and how much he slows down. A steering angle of zero results in the skier moving in a straight line with no turning and no slowing down. When a skier skids around a turn, a reverse camber in the skies causes the skis to self-steer.
Due to the curvature of the skis, the snow impacts the skis at different angles along its length.
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This causes the skis to self steer. To understand the physics behind this self-steering phenomenon, consider an exaggerated representation of a ski shown below. In this illustration the reverse camber of the ski is represented by two flat edges, in which the leading front end of the ski is at a greater angle to the snow than the tailing back end of the ski. Since the leading end of the ski is at a greater angle to the snow than the tailing end, the snow impacts it more head-on, and with greater force.
As a result, a net counterclockwise torque is generated, which causes the skis to self-steer. F S1 is the resultant force of the impacting snow acting on the tail end of the ski F S2 is the resultant force of the impacting snow acting on the leading end of the ski v 1 is the velocity component of the skis along the original direction of motion v 2 is the velocity component of the skis in the sideways direction, in the direction of the turn v is the resultant velocity of v 1 and v 2 Since skidding is a form of snow resistance as described previously which acts to slow the skier down, then v v i.
Note that this neglects any possible change in speed due to a change in the gravitational potential energy of the skier. But regardless, snow resistance results in a lower speed v than would occur if there were no skidding when going around the turn. A skier can enhance the self-steering affect by leaning forward on his skis, and shifting his weight closer to the front of the skis. This results in the leading ends of the skis bending even more, which increases F S2 in the above figure. This in turn causes even more self-steering. The lightly colored line represents the ski before the skier shifts his weight closer to the front.
The skis are also able to self-steer due to the front of the skis being a bit wider than the back. This causes more snow to be plowed at the front leading end of the skis than the back, which results in a greater force being exerted by the snow in the front than the back.
The result of this is a net torque that turns the skis in the same direction as the reverse camber effect does discussed previously. This ski-width effect, however, is not as great as the reverse camber effect in its self-steering ability. In the next section we will look at the forces acting on a skier that is going around a purely carved turn. Force Balance For Skier Going Around A Purely Carved Turn As explained in the previous section, making a purely carved turn is desirable for a skier since it minimizes how much he slows down.
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