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MIT OpenCourseWare http://ocw.mit.edu 6.055J / 2.038J The Art of Approximation in Science and Engineering Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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6.055 / Art of approximation 42 the contact force from the track. In choosing the shape of the track, you affect the contact force on the roller coaster, and thereby its acceleration, velocity, and position. There are an infinity of possible tracks, and we do not want to analyze each one to find the forces and acceleration. An invariant, energy, simplifies the analysis. No matter what tricks the track does, the kinetic plus potential energy 1 2 mv + mgh 2 is constant. The roller coaster starts with v = 0 and height h start ; it can never rise above that height without violating the constancy of the energy. The invariant – the conserved quantity – solves the problem in one step, avoiding an endless analysis of an infinity of possible paths. The moral of this section is: When there is change, look for what does not change. 6.2 Flight How far can birds and planes fly? The theory of flight is difficult and involves vortices, Bernoulli’s principle, streamlines, and much else. This section offers an alternative ap- proach: use conservation estimate the energy required to generate lift, then minimize the lift and drag contributions to the energy to find the minimum-energy way to make a trip. 6.2.1 Lift Instead of wading into the swamp of vortices, study what does not change. In this case, the vertical component of the plane’s momentum does not change while it cruises at constant altitude. Because of momentum conservation, a plane must deflect air downward. If it did not, grav- ity would pull the plane into the ground. By deflecting air downwards – which generates lift – the plane gets a compensating, upward recoil. Finding the necessary recoil leads to finding the energy required to produce it. Imagine a journey of distance s . I calculate the energy to produce lift in three steps: 1. How much air is deflected downward? 2. How fast must that mass be deflected downward in order to give the plane the needed recoil? 3. How much kinetic energy is imparted to that air? The plane is moving forward at speed v , and it deflects air over an area L 2 where L is the wingspan. Why this area L 2 , rather than the cross-sectional area, is subtle. The reason is that the wings disturb the flow over a distance comparable to their span (the longest length). So when the plane travels a distance s , it deflects a mass of air m air ρ L 2 s .
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|{z} 43 6 Box models and conservation The downward speed imparted to that mass must take away enough momentum to com- pensate for the downward momentum imparted by gravity. Traveling a distance s takes time s / v , in which time gravity imparts a downward momentum
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mar17 - MIT OpenCourseWare http:/ocw.mit.edu 6.055J /...

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