The Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence states that the first thing to do after the detection of a possible signal is to
A) send a reply
B) announce the discovery to the public
C) confirm that it is genuine
D) classify it top secret
C
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In the UNS system of material designations, if an alloy designation starts with a “G”, it is a plain-carbon or low-alloy _________.
Fill in the blank(s) with the appropriate word(s).
Which direction do a comet's dust and plasma tails point?
A) straight behind the comet in its orbit B) generally away from the Sun C) perpendicular to the ecliptic plane D) always almost due north
As the rock in this figure falls after being dropped from a great height:
a. its speed increases until it reaches a constant value, but its acceleration decreases gradually to zero. b. its speed continues to increase while its acceleration goes to zero. c. both the speed and the acceleration increase. d. its speed decreases while its acceleration continues to increase. e. its speed slows to zero while its acceleration reaches a terminal value.
A filament of tungsten wire is 0.001 m in diameter, and the filament is heated to 2500 K. The work function of tungsten is 4.5 eV.
(a) Calculate the probability of electrons having sufficient energy to be emitted from the filament. (b) Calculate the emission current density from the tungsten wire. (c) What length of wire can produce a thermionic electron current of 1 A?