Stars much less luminous than the Sun have
A) wider habitable zones, increasing the odds of finding habitable planets and much longer lifetimes allowing life to appear and evolve
B) wider habitable zones, increasing the odds of finding habitable planets but lifetimes too short for life to appear
C) narrower habitable zones, decreasing the odds of finding habitable planets and lifetimes too short for life to appear
D) narrower habitable zones, decreasing the odds of finding habitable planets but much longer lifetimes allowing life to appear and evolve
D
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An aluminum sheet, 0.4-m-tall, 1-m-long, and 0.002-m-thick is to be cooled from an initial temperature of 150°C to 50°C by immersing it suddenly in water at 20°C. The sheet is suspended from two wires at the upper corners as shown in the sketch.
(a) Determine the initial and the final rate of heat transfer from the plate.
(b) Estimate the time required.
(Hint: Note that in laminar natural convection, h ? ?T 0.25)
GIVEN
• A vertical aluminum sheet in water
• Plate dimensions: height (H) = 0.4 m, length (L) = 1 m, thickness (s) = 0.002 m
• Initial plate temperature (Tsi) = 150°C
• Water temperature (T?) = 20°C
• Final plate temperature (Tsf) = 50°C
FIND
(a) The initial and final heat transfer rates
(b) The time required
ASSUMPTIONS
• Constant and uniform water temperature
SKETCH
PROPERTIES AND CONSTANTS
for aluminum at the mean temperature of 100°C
Thermal conductivity (ka1) = 238 W/(m K)
Density (?) = 2702 kg/m3
Specific heat (c) = 896 J/(kg J)
Compared with the buoyant force of the atmosphere on a helium-filled balloon, the buoyant force of the atmosphere on a nearby wood block of the same volume is
A) considerably less. B) considerably more. C) the same.
Components: The x component of vector 
is 5.3 units, and its y component is -2.3 units. The angle that vector 
makes with the +x-axis is closest to
A. 340°. B. 160°. C. 250°. D. 110°. E. 23°.
The phenomenon easily explained only in terms of the particle model of light is
A) reflection. B) refraction. C) photoelectric effect. D) diffraction. E) none of the above