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Name: Patsy
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Question:
Given a perfectly aerodynamic object what is the maximum free fall velocity possible?



Replies:
Hello Patsy,

One of my favorite demonstrations is watching a ball and a feather dropped at the same time in a vacuum. As there is no air resistance on either object they hit the ground at the same time.

As such, it depends upon what you mean by "perfectly aerodynamic." If you mean something that has absolutely no air resistance, then there's only a couple of things that limit how fast an object falls (let us assume that we're talking about a very light object being attracted to a very massive object). In that case it only depends upon the mass of the attracting body and the height from which the object is dropped. In atmosphere there is always going to be some resistance. In general the calculation of air resistance becomes quite complicated, quite quickly.

Very often the resistance will depend upon many different things and will change depending upon how fast the object is traveling (and change in a very nonlinear fashion).

Newton's law still works and is still valid, you just have to include a very complicated set of terms to described the air resistance. Very often we make do with reasonable approximations and descriptions that are valid for a range of interest, but completely wrong outside of that.

best wishes,

Michael


This has no single answer. First, the term "perfectly aerodynamic object" needs an operational definition. What does it mean? In general the "maximum free fall velocity" also called the "terminal velocity" depends on, among other things such as the shape of the object and its mass. A "Google" search on the term: "terminal velocity" gave numerous "hits". Three that seem to provide an understandable explanation are:

http://hyperphysics.phy-astr.gsu.edu/hbase/airfri2.html#c3

http://regentsprep.org/Regents/physics/phys01/terminal/default.htm

http://www.grc.nasa.gov/WWW/K-12/airplane/termv.html

If those do not meet your needs there are other links you can go to for more information.

Vince Calder



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