>> Monday, August 23, 2010
Relax Max continued: If I may have two questions, the second is about terminal velocity. If a large object (a passenger airliner, say) reaches terminal velocity before impact, do smaller (or more aerodynamic) objects whose terminal velocity are higher (I assume) "pass" the airliner on the way down? Or does everything hit the ground at the same time?
Good question(s). As you asked, items don't have the same terminal velocity. The maximum speed an item attains in air depends on many different factors. Surface area, fluid factors, and mass are key factors, but other factors include roughness, shape, initial speed, etc. A man falling, for instance, can change his terminal velocity drastically between falling spread eagle and pulling his limbs in and falling headfirst (as skydivers do to move up and down relative to each other). A man can't change it enough not to be going too fast for landing without a parachute, but that's a different post.
Terminal velocity is effectively the speed an object obtains when the force of gravity is canceled by the opposite drag on an object so that it stops accelerating. If gravity were not involved, there'd be no terminal velocity because drag would just work to make things go slower with no counterforce. (Newtonian physics I can explain if you'd like). Initial velocity makes a difference because it adds a factor beyond gravity (and velocity has an effect on drag).
A debris field is determined by multiple factors as well: initial speed, what caused the initial breakup, and how much and what kind of debris is generated. A biplane, for instance, that lost it's rudder might have a very limited debris field, where as Columbia's debris field extended over several states. Explosions (whether combustive or pressure built) send debris forward and backward, extending the debris field. Flat low mass debris will fall slower that compact debris.
So, to answer the last, everything doesn't land at the same time - except in a vacuum as they demonstrated on Apollo 15.