Expert: Ed - 3/12/2011

Question
Why,when falling to Earth,an object does not feel any acceleration,ie,an accelerometer attached to object will not register any acceleration?And since the accelerometer does not register any acceleration the accelerometer must deduce that it is either stationary or is moving at a constant uniform linear velocity.How can this be?Does this not mean that any object moving under the influence of a gravitational field,say in outer space,but still in the presence of matter,is as far as it is concerned ,is stationary or moving with constant velocity?If whilst driving a car I accelerate at one "G" I will certainly feel this acceleration,what's the difference in these two scenarios?Thanks in anticipation of any response you may choose to make.

Answer
Barry, I am really sorry I didn't answer this faster.  Your question is a good one, and I needed to think it through carefully.  Also I am in the middle of moving, and my entire life is in total chaos.  I have let everything else slide, so why not AllExperts.  I should have told AllExperts I was on vacation or something.

The concepts you should try to understand, and read about in your library, or on wikipedia, or wherever you want, are as follows: free fall, frame of reference, proper acceleration.  Also, remember that acceleration is calculated using vectors, or direction of movement.

You mention being in a car, accelerating at one G.  You would certainly feel that!!!  But remember, in that situation, you would have a total of two Gs pulling on you.  The first G is the Earth's gravity, pulling from below.  The second G is the car's acceleration, smooshing you back in your seat.  Each of these vectors will work on the mass of your body simultaneously.  You will be pulled by pretty powerful forces in perpendicular directions, and the net pull will be highly noticeable.

But if you were an accelerometer inside of a spaceship, or maybe inside a large empty capsule or tank of some kind, or whatever, free falling toward the Earth (or any other planet or gravity source), your frame of reference would be the inside walls of the spaceship, in the same way that your frame of reference is the ground when you are standing on Earth.  Your floor would be falling away from you at exact the same accleration that you would be falling toward it.  As far as an accelerometer would know, there would be no acceleration.

Here is another interesting way to think about this.  Have you heard about the Apollo 15 experiment in which a hammer and a feather were observed to drop at the same speed, in the vacuum and the more relaxed gravity of the lunar surface?
http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html

Well, imagine that your accelerometer is the feather, and the spaceship is the hammer.  If one of them has the other for a frame of reference, why would it think that it is accelerating?  They would be moving at the same speed together, until the instant they both smashed into the ground and broke.  This would apply in any gravity well, not just Earth.

I hope this helps.  Keep Looking Up.
-Ed