# Exam 1 2010 Answer Key - Avian Biology Exam #1 Answer Key...

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Avian Biology Exam #1 Answer Key Spring 2010 1a) (Doug graded) Bernoulli’s law states that static pressure (the force that still air exerts on something) plus dynamic pressure (the force that moving air exerts on something) is a constant. Therefore, where one is higher the other is lower. As air flows over a wing, airfoil or any object where the path of air over the top is longer than under the bottom, there will be lower dynamic pressure and higher static pressure below the object than above it. The net force of static pressure will therefore be upwards, creating lift. You should have provided a diagram similar to those shown in class and available to you on the Powerpoint slides. b) Because the ball is traveling from left to right, air is flowing over the ball from right to left. The path of air is equal on all sides of the ball because the ball is perfectly symmetrical – the ball does not act as an airfoil, in other words. The spin, however, changes the relative speed of air over the top and under the bottom of the ball. Because the ball is rotating clockwise, the top surface of the ball is spinning in an opposite direction (left to right) than the flow of air (right to left). This causes the air flow to slow slightly over the top of the ball. Conversely, the bottom surface of the ball is spinning from right to left, which is in the same direction as the flow of air. This gives the air flow over the bottom of the ball a slight boost, speeding it up slightly. Because dynamic pressure is lower on the top of the ball than on the bottom (air moves slower over the top), static pressure is greater on the top of the ball. c) The pitcher has thrown a “sinker”. The ball will fall more rapidly than expected by the force of gravity alone because the spin causes it to experience “negative lift” (high static pressure on its top side). The ball will not curve to the right or left because it is not spinning to the right or left. (If you answered part c incorrectly but it was logical given an incorrect answer to part b, I generally took off for part b and gave you full credit for part c. In other words, I didn’t hit you twice for the same mistake.) d) The net force on the wing of a plane with wings parallel to the ground should point straight up. You should have drawn a vector (an arrow) on each wing, pointing up. Flow of air from throw Flow of air from throw Pitcher Here Batter Here Direction of spin Direction of spin Flow of air from spin Flow of air from spin Because flow from throw and from spin are in opposite directions, dynamic pressure here will be reduced and static pressure increased.

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e) The net force vector on each wing should be perpendicular to the surface of the wing and 45 degrees to the ground. You should have drawn the vertical component to the net force vector,
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Exam 1 2010 Answer Key - Avian Biology Exam #1 Answer Key...

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