I'm preparing myself for my first physics exam and I have some general problems, I hope you great people will help me (you never let me down so far). Why is momentum conserved in inelastic collision? Is it because we count that "macro" momentum of two clay balls is transferred to "micro" momentum of it's particles ? And I'm highly interested III Newton's law, it seems unbreakable (is there any exceptions), but I'm frustrated when I'm dealing with those collisions (both elastic and inelastic), we have this great talk about force in previous paragraphs of my physics textbook, and suddenly after it force is gone from all calculations, and all we calculate in theory and practice is speed and energy (talking about collisions). Why can't I calculate force, I would love to, if III Newton's says (e.g. inelastic collision) every external force that acts on an object there is a force of equal magnitude but opposite direction which acts back on the object which exerted that external force, IF I understand correctly III law, those two balls acts on each other with same force, and I would like to know what's that force. All I have is change in energy, velocity or momentum of the balls, but F is = m * a , and a = dv/dt, I'm, always short in that dt, even if I wright Fdt=dp, again I have dp but no dt ? I don't get it, I can't calculate force involving collision. + I'm confused with acceleration, if we say that rotational motion is always accelerated (v changes), why is rotational motions so "stabile" (like the Moon rotating around the earth), I know how centrifugal force is making this possible, but is there any example of rectilinear accelerated motion that is so "stabile" and happening "forever". It confuses me because rotational and linear acceleration seems so deferent to me (I would gave them separate names). OK, I've already asked too much , txn p.s. why's speed of sound in air 1000km/h :)?