Astronomers have begun to build instruments to detect "cosmic messengers" that are not forms of light, such as neutrinos, cosmic rays, and gravitational waves, because they
A) provide information about distant objects that we cannot obtain from light.
B) provide higher angular resolution than we can obtain with light.
C) are unaffected by the turbulence of Earth's atmosphere.
D) can allow us to see beyond the horizon of the observable universe.
A) provide information about distant objects that we cannot obtain from light.
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A long wooden rod at 38°C with a 2.5 cm OD is placed into an airstream at 600°C. The heat transfer coefficient between the rod and air is 28.4 W/(m2 K). If the ignition temperature of the wood is 427°C, ? = 800 kg/m3, k = 0.173 W/(m K), and c = 2500 J/(kg K), determine the time between initial exposure and ignition of the wood.
GIVEN
• A long wooden rod is placed into an airstream
• Rod outside diameter (D) = 2.5 cm = 0.025 m
• Initial temperature of the rod (To) = 38°C
• Temperature of the airstream (T?) = 816°C
• The heat transfer coefficient (hc) = 28.4 W/(m2 K)
• The ignition temperature of the wood (TI) = 427°C
• Density of the rod (?) = 800 kg/m3
• Thermal conductivity (k) = 0.173 W/(m K)
• Specific heat (c) = 2500 J/(kg K)
FIND
• The time between initial exposure and ignition of the wood
SKETCH
What kind of variable stars are pre-Main Sequence stars undergoing gravitational contraction and exhibiting erratic changes in their luminosities?
A) R Coronae Borealis B) RR Lyrae C) Cepheid D) T Tauri E) Herbig-Haro
A 30-N box is pulled upward 6.0 m along the surface of a ramp that rises at 37° above the horizontal. How much work does gravity do on the box during this process?
A) -1100 J B) -110 J C) -140 J D) -180 J E) 120 J
Fission fragments tend to be radioactive because they have
A) more protons than neutrons. B) more neutrons than protons. C) too many more protons than neutrons. D) too many more neutrons than protons. E) the same number of neutrons as protons.