Why do stars appear to twinkle?
What will be an ideal response?
In space, the light of the stars appears steady and images are sharp. But in passing through our atmosphere, the narrow shafts of star light are shifted constantly by turbulence, hence the images dance around in the eyepiece for us on the ground.
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What length of a certain metal wire of diameter 0.15 mm is needed for the wire to have a resistance of 15 ?? The resistivity of this metal is 1.68 × 10-8 ? ? m
A) 16 mm B) 16 cm C) 1.6 m D) 16 m E) 160 m
A rem (roentgen equivalent in man) is defined as (the product of)
a. the dose in RBE. b. the dose in roentgen and the RBE factor. c. the dose in rad times the dose in roentgen. d. the dose in rad and the RBE factor. e. the dose in rad and energy of radiation.
Addition by 1. Components: Vector 
= 4.00 m points eastward and vector 
= 3.00 m points southward. The resultant vector 
+ 
is given by
A. 5.00 m at an angle of 36.9° south of east. B. 5.00 m at an angle of 53.1° south of east. C. 5.00 m at an angle of 71.6° south of east. D. 5.00 m at an angle of 18.4° south of east. E. 5.00 m at an angle of 26.6° south of east.
In a long, 30-cm square bar shown in the accompanying sketch, the left face is maintained at 40°C and the top face is maintained at 250°C. The right face is in contact with a fluid at 40°C through a heat transfer coefficient of 60 W/(m2 K) and the bottom face is in contact with a fluid at 250°C through a heat transfer coefficient of 100 W/(m2 K). If the thermal conductivity of the bar is 20 W/(mK), calculate the temperature at the 9 nodes shown in the sketch if the temperature distribution on the top surface of the bar varies sinusoidally from 40°C at the left edge to a maximum of 250°C in the center and back to 40°C at the right edge.
GIVEN
Square bar with one surface at fixed temperature, one surface with a specified temperature
distribution, and two surfaces with convective boundary conditions
FIND
(a) Temperature at 9 shown nodes
SKETCH
