Originally Posted by CarvingFan
...While skiing there are two forces at work, gravity and vis inertiae. We discern two aspects of mass, the weighting mass and the inertial mass. The weighting mass depends on gravity, the inertial mass on acceleration...At last it means a mass wants to keep speed and direction. Every change causes a force. The centrifugal force we experience during skiing a turn is a direct consequence of this force. ...
PS: For the physicist amoung us, yes, this is not as exact as possible. That would be too much. I've used the term because I don't know a better and more common word for it and I had to serve my sentence to explain.
CarvingFan - Because you are new to this discussion group, you probably aren't aware that a few weeks ago we just concluded a very lengthy discussion (300+ posts in one thread) of centrifugal vs centripetal forces, "real" vs "pseudo forces", etc. Because of the time and energy this discussion took, I am very hesitant to correct some seemingly minor (but actually fundamentally important) points in the above post. However, because they could mislead / confuse people, I am going to, but I don't intend to enter into a lengthy discussion of these points. If you disagree with what I am about to say, I would simply recommend that you consult a standard textbook of mechanics / dynamics."... The weighting mass depends on gravity, the inertial mass on acceleration ..."
Your terminology is non-standard and possibly confusing. I have never heard or seen the phrase "weighting mass" used.
Objects have only one mass. Gravity acts on that mass to produce the effects and sensations of weight, not the other way around. It is the same for the second part of your sentence - forces act on that (single) mass to produce acceleration. The way you phrased your sentence, it sounds like if there is no gravity, there is no (weighting) mass, and clearly this is not true. Mass exists with or without gravity, with or without acceleration."... Every change causes a force ..."
This one seeming innocuous little sentence (or its equivalent) can cause a huge ammt of confusion among lay people and is essentially at the heart of the lengthy thread that I mentioned above.
Usually, people say, "every force causes a change in velocity". What this means is that when viewed from an external, non-accelerating coordinate system
, forces cause changes in velocity (ie, accelerations).
You said something different: "Every change in velocity causes a force". While this can be true, this only happens when changes in velocity are not visible to an observer (ie, an observer inside a closed box being pushed from the outside). This is what is called an non-inertial (or accelerating) frame of reference, and can indeed can cause this observer to experience very real forces, but these can be difficult for lay people to understand, and hence are sometimes called pseudo-forces.
Unless really needed, analysis of dynamics in such accelerating frames of reference forces is probably best not done. Unfortunately, skiers making turns are always in an accelerating frame of reference and so the issue of what they experience in such a frame of reference must be dealt with. As you point out, this (ie, centrifugal
force) is why they feel like they are being thrown to the outside of the turn. A stationary observer looking at the skier from the side of the hill sees the snow pushing on the skier making him/her turn (ie, the centripetal
force). Enough said on this. For more details, you may want to take a look at the old thread on this subject.
Tom / PM