It is a riddle.
It is the strongest force,
to resist your force,
and yet it is not a force.
It never accelerates,
but it is always moving.
It never rests, and
it never gets tired.
It has been defined many ways,
but no one can quite capture it.
What is it?
(In reply to re(3): ... (physics 101)
I just get so much joy out of trying to figure out JC's next move...
Amon - Centrifugal force is the imaginary one - there is no such thing - it is the result of looking at a system from within a particular frame of reference. The actual acceleration is called centripetal acceleration, and is the same magnitude but opposite direction of centrifugal acceleration. It is what holds something in "orbit" rather than letting it fly in a straight line away.
Now, the earth changes speed all the time. We would be going at a constant speed if the earth were travelling in a circular orbit around the sun (and our solar system were the extent of the universe, but lets just leave that out, shall we), however the orbit is actually elliptical, and so the earth is travelling slower (sigh, with respect to the sun) when it is further away from the sun than it does when it is closer to the sun. So even ignoring the angular compnent of velocity, the earth must accelerate :)