heedm, thanks for your reply.. .. .You said...There are not three distinct frames of reference. A frame of reference is something you purposely or inadvertently choose when you make a measurement. That reference frame is either inertial or non-inertial. There are an infinite number of choices of each.. .. .I generally agree with you, and there are an infinite number of choices for each type of reference frame. It might be fair to say however that there are 2 measurement types of reference frames, and one discreet object reference frame.. .. .You said...A free body diagram is not a reference frame. It is a simplification of reality chosen to illustrates something. When you make some sort of measurement on your free body diagram, you are selecting a reference frame. That frame is inertial or non-inertial.. .. .One HAS to take measurements of the forces acting on a discreet body, from either an inertial or non-inertial reference frame. But once in the free body "perspective" those forces are summarized as vectors and magnitudes (and sometimes with changing vectors and values over discreet periods of time, if that's part of a computer simulation).. .. .You said...Centrifugal and centripetal forces are the same magnitude and they point in opposite directions, but it is wrong to call them equal and opposite. Very very wrong.. .. .I beg your pardon, can you say that again?. .. .You said...The fact is, centrifugal forces can be observed in one reference frame where a centripetal force is produced in a different one. Because they can't both be observed equally and oppositely in one reference frame, they are not equal and opposing forces.. .. .Both forces exist due to acceleration, centrifugal does not exist with centripetal. What prevents them from both being observed and measured in a rotating reference frame?. .. .You said..."You’ll note that in most cases the “drag” force does not exist if the “thrust” force does not exist." Not at all. If this were true then gliders wouldn't have any drag. Drag is a force that is dependant on velocity, not on thrust.. .. .Alright, I knew when I used the powered airplane example that I was referring to level flight, and the statements I made are valid in regards to maintaining constant speed level flight. The glider diagram has only 3 forces, but that wouldn't have been helpful in my explanation. In the glider, gravity provides the force to accelerate the air molecules out of the way of the passing airplane. Just to further clarify, I was referring to the aerodynamic drag, not the induced drag.. .. .You said...Last point. Why is an apparent force called a force at all? I guess the simplest answer is "if it talks like a duck and walks like a duck, call it a duck.". .. .Here's my whole problem. If if "feels" like a force (walks, talks, and quakes like a force) it must be a force. But wait, it only seems like a force (even though it walks and talks and quakes like one), but really it's not, so in reality its only "apparent". Honestly, is there confusion here or what?. .. .(edited to add the last point). . . . <small>[ 18 March 2002, 23:04: Message edited by: Flight Safety ]</small>