Astronomers refer to the dark center of a sunspot as a(n) ____
a. spicule
b. flare
c. penumbra
d. umbra
e. eye
d
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Projectile Motion: A wind farm generator uses a two-bladed propeller mounted on a pylon at a height of 20 m, as shown in the figure. The width of the pylon is very narrow, and the length of each propeller blade is 12 m. A tip of the propeller breaks off just when the propeller is vertical. The fragment flies off horizontally, falls, and strikes the ground at point P with negligible air resistance. Just before the fragment broke off, the propeller was turning uniformly, taking 1.2 s for each rotation. How far is point P from the base of the pylon?
A. 120 m B. 130 m C. 140 m D. 150 m E. 160 m
Order the interior layers of the Sun from hottest to coldest.
A) radiation zone, core, convection zone, photosphere B) core, radiation zone, convection zone, photosphere C) photosphere, convection zone, radiation zone, core D) photosphere, convection zone, core, radiation zone
Meteorites are important because
A) some come from the Moon and Mars, as well as the asteroid belt. B) they contain pristine material from the solar nebula. C) large ones may cause mass extinctions. D) All of the above are true. E) None of the above is true.
Pencil "lead" (graphite) is made primarily of
a. oil. b. PbO2. c. carbon. d. Pb.