Why should galaxy collisions have been more common in the past than they are today?
A) Galaxies were closer together in the past because the universe was smaller.
B) Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars.
C) Galaxies were more active in the past and therefore would have collided with each other more frequently.
D) Galaxies were much bigger in the past since they had not contracted completely.
A
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The best way to increase the calculated fatigue-crack propagation cycles to failure of a part is
(a) To select a material with a high KIc (b) To ensure that the part has a very small initial crack length with nondestructive testing equipment. (c) To polish the surface of the part (d) To case-harden the part
In light of what we know about geological activity on the terrestrial worlds, why might it seem surprising to find geological activity on jovian moons?
a. because some of them have orbital resonances with other moons b. because they rotate slowly c. because they are relatively small in size d. because they are moons, not planets e. because they are far from the Sun
If a satellite's radial velocity is zero at all times, its orbit must be
A) parabolic. B) elliptical. C) circular. D) geosynchronous.
Which of the following air pollutants is emitted mainly by the automobile?
a. particulates b. carbon monoxide c. sulfur dioxides d. hydrocarbons e. nitrogen oxides