Problem 3.61

# Problem 3.61 - First the horizontal projected area of the...

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Problem 3.61 [Difficulty: 1] Given: Description of car tire Find: Explanation of lift effect Solution: The explanation is as follows: It is true that the pressure in the entire tire is the same everywhere. However, the tire at the top of the hub will be essentially circular in cross-section, but at the bottom, where the tire meets the ground, the cross section will be approximately a flattened circle, or elliptical. Hence we can explain that the lower cross section has greater upward force than the upper cross section has downward force (providing enough lift to keep the car up) two ways.
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Unformatted text preview: First, the horizontal projected area of the lower ellipse is larger than that of the upper circular cross section, so that net pressure times area is upwards. Second, any time you have an elliptical cross section that's at high pressure, that pressure will always try to force the ellipse to be circular (thing of a round inflated balloon - if you squeeze it it will resist!). This analysis ignores the stiffness of the tire rubber, which also provides a little lift....
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