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th 8 Grade Science Name: __________________________ Period: ________ Date: _____________________ Station 1: Science of Skateboarding: The Ollie 1. Search “popping and jumping tips for skateboard ollies” and select the ehow.com video. This site demonstrates and explains the technique for performing a skateboarding move called the ollie, named after Alan “Ollie” Gelfand, a skateboarder for the 1970‟s. Watch the video clip carefully and read through the instructions, then we will analyze the move and see how we can apply the rules of science to understand it. 2. Read the following to get a better picture of what happens in an ollie: (thanks to the Exploratorium Museum website) Just before a skater performs an ollie, there are three forces acting on the skateboard. One of these forces is the weight of the rider, pushing down on the board. Another is the force of gravity on the board. Finally, the ground exerts a force upward on the board. These three forces balance out to zero. st With no net force, the skateboard doesn‟t accelerate, but rolls along at a constant speed (Newton‟s 1 Law of Motion). Notice how the skater crouched down when he first began? A low center of mass will be crucial to getting a high jump. Try it yourself: Stand perfectly straight and try jumping without bending your knees or using your arms… you didn‟t get very high, did you? Now let‟s follow the changing forces that go into making an ollie. The skater accelerates himself upward by explosively straightening his legs and raising his arms. During the jump, his rear foot exerts a much greater force on the tail of the board than his front foot does on the nose, causing the board to pivot counterclockwise about the rear wheel, so the front end goes up in the air (the skateboard is acting like a lever). As the tail strikes the ground, the ground exerts a large upward force on the tail. The result of this upward force is that the board bounces up and begins to pivot clockwise, this time around its center of mass. With the board now completely in the air, the skater slides his front foot forward, using the friction between his foot and the rough surface of the board to drag the board upward even higher. The skater begins to push his front foot down, raising the rear wheels and leveling out the board. Meanwhile, he lifts his rear leg to get it out of the way of the rising tail of the board. If he times this motion perfectly, his rear foot and the rear of the board rise in perfect unison, seemingly „stuck‟ together. The board is now level at its maximum height. With both feet touching the board, the skater and the board begin to fall together under the influence of gravity. Gravity eventually wins out and the skater bends his legs to absorb the impact of the landing. Questions: 1. What are the three forces acting on the skateboard before the move? 2. How is crouching down helpful in skateboarding? 3. How does the skateboarder use friction during the ollie? **** Bonus **** How does the skateboard act as a lever at times? Station 2: Science of Skateboarding: A skateboarder launches straight into the air from the top of a ramp. Seeming to hang in place for just a moment, he turns in midair and directs himself back down the ramp. Skaters call this maneuver a frontside 180. One of the basics of physics is the law of conservation of angular momentum. It says: If you‟re rotating, you‟ll keep rotating unless a twisting force acts to stop you. Likewise, if you‟re not st rotating, you can‟t rotate unless a twisting force starts you rotating (kind of like Newton‟s 1 law with a twist). Getting back to the skater, there‟s just one more important detail. If you‟re in midair, the only force that can act on you is gravity. Gravity can‟t make you rotate; it can only make you fall. So the question is, how does a skater go from not rotating to rotating? Try the following with a partner. 1. Find a clear area and stand facing your partner. 2. Position a chair against a wall or cabinet. Be sure you have room to jump off of the chair without hitting anything. Stand on the chair, jump into the air. Just as you become airborne, your partner should o point either to your left or your right. Now, while you‟re still in the air, quickly turn your body 90 in the direction that your partner pointed. 3. This isn‟t easy, so practice it several times to make sure that you are turning in the air. If you managed to do this, you‟ve just turned in midair while keeping your angular momentum constant at zero. The random choice of your partner guarantees that you didn‟t get your rotation by pushing off from the ground. So where did you get it? From your upper body. As you rotate your legs o 90 beneath you, your arms and torso rotate in the opposite direction. You probably found yourself naturally sticking your arms out as you turned. The large rotation of your legs is cancelled by a small rotation of your outspread arms. Since the two rotations cancel, angular momentum stays at zero, and the law of conservation of angular momentum is satisfied. Skateboarders turn in midair by twisting their arms and legs in opposite directions. Upon landing, a skater can use the friction between his or her feet and the skateboard to twist the upper body back into alignment. Questions: 1. What does the Law of Conservation of Momentum say? 2. If you were to perform a frontside 180, how do you rotate in midair? 3. What role does friction have in doing a frontside 180? 4. What role does gravity play in doing a frontside 180? Station 3: How Bike Brakes Work: All brakes, regardless of the mechanism, share one thing in common: They increase the amount of friction on the wheel allowing the rider to slow or stop. The coaster brake is still widely used throughout the world and appears in utility bikes, children‟s bicycles and tricycles. The coaster brake works by reversing the motion on the pedals. The brake mechanism is inside the hub of the wheel and pushes outward on the hub, creating friction and slowing the bike. This brake is particularly strong and tends to „lock up‟ or skid the rear wheel when applied. The most popular brake for road and mountain bicycles is the caliper rim brake. The cyclist engages these brakes by pulling on levers which pull cables, forcing pads or shoes against the inner rim of the front or rear wheel. Caliper brakes are lightweight and inexpensive, but they are not without problems. During wet weather it may take twice the distance to stop as it does in dry. The water acts as a lubricant on the sides of the rims. Also, during very long downhills the rims can heat up, even to the point of melting a hole in the tire‟s inner tube! When using caliper brakes, riders are advised to place gentle pressure on the brakes and pump in a controlled fashion. Controlled pumping will help brakes perform better, particularly in wet weather, by removing some of the excess liquid from the rims. In addition, pumping the brakes will ensure the rider does not lock up the brakes, causing the rider to lose control over the bicycle. (information from the Exploratorium Museum, San Francisco, CA) Questions: 1. Contrast (tell the differences between) the coaster brake and the caliper brake. 2. Explain how friction works in braking systems. 3. What are two problems with caliper brakes? 4. How does pumping brakes help the brakes work better? 5. What would the best bike brake pad material be made of and why? Station 4: Why Are Hockey Pucks Kept Frozen? Procedure: 1. Obtain the following: 2 rubber balls: one at room temperature, one from the freezer (kept in insulated bag until you are about to use it), meter stick. 2. Hold meter stick vertically in the air with zero at ground level. 3. Drop room temperature ball from desk height, observe and record the height at which it bounces. 4. Repeat two more times, record results in table below. 5. Calculate average height of bounce, record in table below. 6. Repeat steps 3 – 5 with ball from freezer, making sure you do it quickly so the ball doesn‟t warm up too much. Observations: Trial # Room temperature ball Ball from Freezer Bounce Height (cm) Bounce Height (cm) 1 2 3 Average Questions: 1. Why do hockey teams put pucks in the freezer before play? 2. Give a possible explanation for why a cold ball bounces differently than a warm ball.
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