A ceiling fan is turned on and reaches an angular speed of 120 rev/min in 20 s. It is then turned off and coasts to a stop in an additional 60 s. The ratio of the average angular acceleration for the first 20 s to that for the last 60 s is which of the following?
a. 3
c. 0.3
b. –3
d. –0.3
B
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A 75.0 kg ladder that is 3.00 m in length is placed against a wall at an angle theta. The center of gravity of the ladder is at a point 1.2 m from the base of the ladder. The coefficient of static friction at the base of the ladder is 0.400. There is no friction between the wall and the ladder. What is the minimum angle the ladder makes with the horizontal for the ladder not to slip and fall?
A. 35 degrees
B. 45 degrees
C. 53 degrees
D. 60 degrees
E. 65 degrees
At temperatures of ________ K, photons can split apart nuclei until only protons and neutrons are left in photodisintegration
A) ten million B) 100 million C) one billion D) ten billion E) one hundred billion
Super-Earths are:
A) planets that are most earthlike, likely to harbor life. B) planets that are a few times the mass of the Earth. C) planets that have earthlike masses, but orbit much closer to their star than the Earth does to the Sun. D) any rocky (or terrestrial) planet. E) Earth-mass planets that are much lower in density than the Earth, giving them larger radii.
U-235 and Pu-239 are both
A) radioactive isotopes. B) fissionable isotopes. C) heavier than lead. D) all of the above E) none of the above