This chapter discusses the "critical density" of the universe. More generally, when we say that something is a "critical value" in science, we mean
A) a number that signifies some boundary or change.
B) a value that has been heavily criticized.
C) an idea, representing a value system, that is very important.
D) any number that has been very precisely measured.
A) a number that signifies some boundary or change.
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A 900-kg car traveling east at 15.0 m/s collides with a 750-kg car traveling north at 20.0 m/s. The cars stick together. Assume that any other unbalanced forces are negligible
(a) What is the speed of the wreckage just after the collision? (b) In what direction does the wreckage move just after the collision?
A 10-m plank is being moved by rolling it over two cylindrical logs placed 2 m from either end of the plank. As the plank is pushed, the logs roll on the ground without slipping, and they do not slip with respect to the plank
How far can the plank be moved before the rear log reaches the end of the plank?
An uncharged pith ball is suspended by a nylon fiber. When a positively-charged rubber rod is brought nearby, the pith ball
A) moves toward the rod. B) is unaffected. C) moves away from the rod. D) none of the above
Where did Thomson's model place electrons in the atom?
a. Studded throughout a positively-charged material. b. Rotating around a positively-charged core. c. Uniformly distributed in the atom. d. Clustered in the center of a positively-charged sphere. e. At the two magnetic poles of the atom.