At about what frequency is Earth struck by a meteorite large enough (about 10 km in diameter) to cause mass extinctions?
A) about every 1 million years
B) about every 10 million years
C) about every 100 million years
D) This has yet to occur on Earth.
Answer: C
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A radar pulse is bounced off the Moon at a distance of 400,000 km. What is the time from the transmitting of the pulse until its return?
a. 1.28 s c. 2.67 s b. 2.56 s d. 3.84 s
Two parallel disks shown in the figure have emissivity of 0.6. If one of the disks is at 200ºC and the other one is at 30ºC, (a) determine the net heat transfer between the two disks if the surroundings re-radiate heat and (b) determine the net heat transfer between the two disks if the surroundings are a black body at 20ºC.
Consider fully developed laminar flow of a fluid inside a wide rectangular duct with both the upper and lower surface at uniform surface temperature, as schematically shown in the figure below. The effect of the two sides of the duct is neglected (W>> H), and at a location far from the entrance the velocity distribution is assumed to be uniform, u(x) = Cu whereas the temperature distribution has a parabolic profile given by T(x)= Ts + CT (1 –(2x/H)2). From the given velocity and temperature distributions, calculate the Nusselt number, which is given by its definition as where Tb is the average bulk temperature of the flowing fluid at the specified location, and k is the thermal conductivity of the fluid.
GIVEN
Fully developed laminar flow of fluid inside rectangular duct
Uniform upper and lower surface temperature
Uniform velocity distribution u(x) = Cu
Parabolic temperature distribution T(x)= Ts + CT (1 –(2x/H)2
FIND
Nusselt number
SKETCH
Which of the following statements is true?
A. Venus rotates almost precisely 5 times relative to Earth between closest approaches. B. Venus's thick atmosphere causes strong tidal braking. C. Venus has a longer day than its year. D. All of these choices are correct.