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final08

final08 - Final Fill in Bubble A in the Test Form box 1 An...

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Physics 17 2008 Final Fill in Bubble A in the Test Form box. 1. An ice skater pulls her arms in to spin faster. Of these four quantities–angular velocity ω , angular momentum L , kinetic energy K , and rotational inertia I –which quantity decreases ? (a) ω (b) L (c) K (d) I (e) more than one decreases 2. An ice skater pulls her arms in to spin faster. Of these four quantities–angular velocity ω , angular momentum L , kinetic energy K , and rotational inertia I –which quantity increases ? 3. Select the best answer to the warmup question, “How do athletes in extreme sports get so high in the air?” (a) They usually are hunched over or crouched when they jump. This is because you need a low center of mass to get high. When athletes get ready to do a jump and get high in the air, you do not see them on their boards or bikes in a straight position but they are usually crouching or hunched. In skateboarding once in the air, the skater slides his front foot closer to the edge of the board using the friction between his foot and the surface of the skateboard to drag the board up even higher. Also, the athletes build up acceleration; the greater the acceleration, the higher the jump. (b) The skater’s weight, the force of gravity on the board, and the force that the ground exerts on the board are the three forces that act on a skateboarder. All of these forces add up to zero, allowing the skater to skate at a constant speed, unless another force is exerted upon the skater. (c) Athletes in extreme sports like skateboarding and BMX need to achieve a large velocity at the bottom of the ramp, so that they will have larger kinetic energy. On their way up the ramp, their kinetic energy converts to potential energy, and according to the formula, PE = mgh , and mgh = 1 2 mv 2 , a larger velocity on the bottom of the ramp means a greater height. Velocity is the most important part of achieving great height. (d) Athletes in extreme sports manage to jump so high because their bikes and skateboards are lightweight, so the force required to lift their bikes into the air isn’t that much. (e) In sports like skateboarding and BMX, athletes are able to get so high because ramps enable them to have double the terminal velocity. For example, when Tony Hawk is skating off a ramp, he builds up his velocity very high then when he gets on the ramp, he ollies off right before the edge of the ramp. The ramp already has a launch angle with horizontal and vertical vectors. Because he is also jumping (ollie), he adds another verticle vector to reach maximum height. 1

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F 1 F 2 F 3 F 4 The diagram on the right shows the forces that act on the snowboarder when he is going straight down the hill in the picture on the left. (Note that the athlete’s boots are firmly strapped to his board so the athlete and board act as a single unit.) F 1 and F 3 act where the board touches the snow and F 2 acts primarily on the snowboarder’s jacket. The next five questions refer to this figure.
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