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讲座12：非保守力-阻力-空气阻力-终端速度

Lecture 12: Non-Conservative Forces - Resistive Forces - Air Drag - Terminal Velocity
课程网址: http://videolectures.net/mit801f99_lewin_lec12/  
主讲教师: Walter H. G. Lewin
开课单位: 麻省理工学院
开课时间: 2008-10-10
课程语种: 英语
中文简介:
** 1。阻力和阻力：**阻力有一个粘性项，其速度是线性的;它对温度敏感，反映了介质的粘性。此外，它们的压力项与速度平方和流体密度成正比。电阻力始终与速度相反。随着速度的增加，阻力增大。因此，空气中（或液体中）的下落物体将达到最终速度。 ** 2。两个政权和临界速度：**阻力或阻力有两个项;粘性和压力条件。它们在“关键”处的大小相等。速度。在远小于此的速度下，球形物体（所有具有相同密度）的终端速度随着物体的半径平方而增加。在远大于临界速度的速度下，压力项占主导地位，并且终端速度随着半径的平方根而增加。 ** 3。在糖浆中使用钢球进行测量：**在Karo玉米糖浆中滴下小球轴承。 Lewin教授解释了为什么滚珠轴承的终端速度会随着半径的平方而变化。他进行测量以验证这一点。 ** 4。在眨眼间达到终端速度：**当球轴承掉入糖浆中时，它们的速度首先会增加。结果表明它们的终端速度非常快。 ** 5。空气阻力和压力项：**几乎所有从相当高的空气（雨滴或天空潜水员）落入空气中的物体的空气阻力由压力项决定。因此，终端速度随着具有给定密度的球半径的平方根而增加。如果要计算达到此终端速度所需的时间，则必须同时包含v和v平方项。 Lewin的研究生Dave Pooley用数字方法解决了这个等式，然后用一个从大约3米高处掉落的气球（充满空气）进行测量。 ** 6。空气阻力示例的数值计算：**从475米（帝国大厦）的高度坠落的鹅卵石，在5-6秒内达到每小时约75英里的终点速度。 Lewin教授还讨论了空气阻力对苹果掉落过程中早期进行的定量实验的贡献。 ** 7。阻力和轨迹：**空气阻力将导致物体在空中抛出的不对称轨迹。对于网球和对于相同半径的聚苯乙烯泡沫塑料球，阻力大致相同，但是阻力对于较轻的球的轨迹具有更显着的影响。
课程简介: **1. Resistive and Drag Forces:** Resistive forces have a viscous term that is linear in velocity; it is temperature sensitive and reflects the stickiness of the medium. In addition they have a pressure term that is proportional to the speed squared and the fluid's density. Resistive forces are always in the direction opposite the velocity. The resistive force grows as the speed increases. Therefore, a falling object in air (or in a liquid) will reach a terminal velocity. **2. Two Regimes and the Critical Velocity:** The drag, or resistive force has two terms; the viscous and pressure terms. They are equal in magnitude at a "critical" speed. At speeds much smaller than this, the terminal speed for spherical objects (all with the same density) increases as the radius squared of the objects. At speeds much larger than the critical speed, the pressure term dominates and the terminal velocity increases as the square root of the radius. **3. Measurements with Steel Balls in Syrup:** Small ball bearings are dropped in Karo Corn Syrup. Professor Lewin explains why the terminal velocity of the ball bearings will vary with the radius squared. He conducts measurements to validate this. **4. Reaching Terminal Velocity in the Blink of an Eye:** When the ball bearings are dropped in the syrup, their speeds at first increase. It is shown that they reach their terminal velocity very fast. **5. Air Drag and the Pressure Term:** The air drag on almost all objects that fall in air from a considerable height (raindrops or sky divers) is dominated by the pressure term. Thus the terminal speed increases with the square root of the radius of spheres with given density. If you want to calculate the time it takes to reach this terminal speed, you have to include both the v and v-squared terms. Lewin's graduate student, Dave Pooley, solved the equation numerically, and a measurement is made with a balloon (filled with air) that is dropped from a height of about 3 meter. **6. Numerical Calculations of Air Drag Examples:** A pebble, dropped from a height of 475 meters (the Empire State Building), reaches a terminal speed of about 75 miles per hour in 5-6 seconds. Professor Lewin also discusses the contribution of air drag to the quantitative experiments done earlier in the course with falling apples. **7. Resistive Forces and Trajectories:** Air drag will result in an asymmetric trajectory for an object thrown up in the air. The resistive force is about the same for a tennis ball as for a styrofoam ball of the same radius, but the resistive force has a much more dramatic effect on the lighter ball's trajectory.
关 键 词: 讲座12; 阻力; 终端速度
课程来源: 视频讲座网
最后编审: 2020-07-05：liush
阅读次数: 74

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