>Hi Hilmar,
>...
>are you sure about this? I remember being on an amusement park ride that stands us all up at the outside edge, then it begins to spin. The effect is that people get 'stuck' to the back wall. We certainly did not all end up on top of each other in the middle < s >.
>
>Also, I do remember discussions of centrifugal force in physics classes.
>
>cheers
> SNIP

The following is more or less adapted from the physics book I studied in high school.

Let's consider a movement in a straight line first, since this is a simpler case. You are standing on the floor in a train; the train is accelerating. You feel a force that seems to pull you backwards. If the floor is slippery, you will seem to be accelerating backwards - but this is in relation to the train. For an outside observer, however, you are not accelerating backwards, and no force is involved in you being "pulled back", since your natural tendency, due to inertia, is to stay in one place.

Now, if you hold fast to something on the train, you will be accelerated together with the train. This is due to an unbalanced force that pulls you forward. If there really was an equal-but-opposing force pulling you backward (as it may seem), you would not accelerate, due to Newton's first and second laws.

Therefore, we must conclude that the force you feel, which pulls you backward, is an apparent force, or a ficticious force - a fiction which is only required if you assume the train to be an "intertial reference frame", that is, if you assume that it is not accelerating.

If you agree so far, I can proceed - in another message - to compare the case of a circular movement.