Which of the following models best explains why our galaxy has spiral arms?
A) No model can explain the existence of the arms, which rotate with the galaxy like the fins of a giant pinwheel toy.
B) The spiral arms were imprinted on the galaxy at its birth. Ever since, like a coiling rope, the spiral arms have been wound tighter with each galactic rotation.
C) The spiral arms are composed of groups of stars that are bound together by gravity and therefore always stay together as the galaxy rotates.
D) The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy.
D
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The Sun appears to rotate only because the Earth is revolving about the Sun
Indicate whether the statement is true or false
Hot water is used to heat air in a double pipe heat exchanger as shown in the following sketch. If the heat transfer coefficients on the water side and on the air side are 550 W/(m2K) and 55 W/(m2 K), respectively, calculate the overall heat transfer coefficient based on the outer diameter. The heat exchanger pipe is 5 cm, schedule 40 steel (k = 54 W/m K), with water inside but assume that a fouling factor of 0.173 (m2 K ) /kW based on the tube outside diameter has developed over time.
GIVEN • A double pipe heat exchanger with water in inner tube and air in the annulus • Heat transfer coefficients ? Water side h i= 550 W/(m2 K) ? Air side h o= 55 W/(m2 K) • Inner pipe: 2 in., schedule 40, made of steel • Pipe thermal conductivity (k) = 54 W/(m K) • Fouling factor (Rd) = 0.173 (m2 K )/kW= 1.73*10-4 (m2 K )/kW, based on the tube outside area FIND • The overall heat transfer coefficient based on the outer diameter (Uo) ASSUMPTIONS • Uniform heat transfer coefficients • The given fouling factor is based on the outer tube diameter (Do)
Approximately how long after the Big Bang did the era of nucleosynthesis end, marking the time when the basic chemical composition of the universe was determined?
A) 10-10 second B) 0.001 second C) 5 minutes D) 380,000 years E) 5 million years
A flatbed truck is carrying a 20.0-kg crate along a level road. The coefficient of static friction between the crate and the bed is 0.400, and the coefficient of kinetic friction is 0.300
What is the maximum acceleration that the truck can have if the crate is to stay in place relative to the truck? A) 8.00 m/s2 B) 3.92 m/s2 C) 196 m/s2 D) 78.5 m/s2 E) 7.84 m/s2