I have three unrelated questions about concepts in physics. I would appreciate any answers you guys can provide 1) Does the definition of "fluids" include something like fine sand? Or numerous small pebbles? 2) Suppose you're sealed inside a large box in outer space, with whatever you'd like to bring along. Is it possible, by doing experiments inside of the box, but without looking outside of it, to determine whether the box and all of its contents are being accelerated by some external force? 3) My book describes the "Rotor", an amusement park ride where patrons enter a large mechanical cylinder. The cylinder begins to spin quickly, and the riders find themselves pinned against the walls of the cylinder. Pinned strongly enough, in fact, that once the Rotor is up to speed, the floor can drop away and the riders will not fall, through a combination of static friction and a normal force from the walls of the Rotor. The way my book sets up the problem, the static frictional force is directed only to oppose gravity (straight up), and does not have a component tangent to the Rotor's movement. Is that right? I believe that it is not, because if that were true, the Rotor would be unable to change the rider's angular speed by spinning faster or slower.