Lecture%2019%20Work%20and%20Transportation-slides

Lecture%2019%20Work%20and%20Transportation-slides - D EAL...

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Unformatted text preview: D EAL WITH D ECEMBER N OW ! A Final Exam Preparation Workshop for Science Students Register at: https://secure.students.ubc.ca/workshops/study.cfm This workshop is about study techniques, not content. Today Air drag Energy and fuel consumption in transportation 3 Air Drag Due to collision of an object with air molecules. Direction is opposite to the velocity of the object. 4 Air Drag Due to collision of an object with air molecules. Direction is opposite to the velocity of the object. Magnitude depends on size and shape of the object. A: cross sectional area (front surface), Density of air: = 1.29 kg/m 3. Drag coefficient contains aerodynamics of shape. Typically, C D ~ 0.5. Formula applies for most objects if not too small or too fast. 2 2 1 v A C D D Head Wind and Tail Wind Why is it harder if you ride into a head wind? v air v bike v bike v air Head Wind and Tail Wind Why is it harder if you ride into a head wind? When the air is already moving, we need to consider the relative speed v rel between air and the bike: Head wind: v rel = v air + v bike Think of a two step process: air molecules are stopped first, then accelerated in opposite direction. Both steps require energy. v air v bike v bike v air 2 2 1 rel D v A C D Head Wind and Tail Wind Why is it harder if you ride into a head wind? When the air is already moving, we need to consider the relative speed v rel between air and the bike: Head wind: v rel = v air + v bike Think of a two step process: air molecules are stopped first, then accelerated in opposite direction. Both steps require energy. Tail wind: v rel = v air- v bike Molecules already have speed in the direction of the bike. Less energy required to get them up to the same speed. v air v bike v bike v air 2 2 1 rel D v A C D Air Drag: Falling Objects For a falling object (with air resistance), which of these statements is incorrect ? A. During the fall the acceleration decreases until it is zero. B. Acceleration and velocity are in the same direction. C. The drag force increases while a > 0. D. The velocity decreases until terminal velocity is reached. E. Then, as a = 0, the velocity remains constant. Air Drag: Falling Objects For a falling object (with air resistance), which of these statements is incorrect ? A. During the fall the acceleration decreases until it is zero....
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Lecture%2019%20Work%20and%20Transportation-slides - D EAL...

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