Wave Characteristics: What is the frequency of 20-mm microwaves? (c = 3.0 × 108 m/s)
A. 100 MHz
B. 400 MHz
C. 15 GHz
D. 73 GHz
Answer: C
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When we glimpse the chromosphere at the start and end of totality, its color is:
A) green (the famous flash). B) yellow, like the photosphere below it. C) red, due to ionized hydrogen at lower pressure. D) blue, due to the ionization of nitrogen by the magnetic fields. E) white from the moonlight.
An unusual lightning strike has a vertical portion with a current of ?400 A downwards. The Earth's magnetic field at that location is parallel to the ground and has a magnitude of 30 ?T. In N, the force exerted by the Earth's magnetic field on the 25 m-long current is
A. 0. B. 0.012 N, East. C. 0.012 N, West. D. 0.30 N, West. E. 300 N, East.
If you run a complete loop around an outdoor track (400 m) in 100 s, your average speed is
A) 40000. m/s. B) 4.0 m/s. C) 4.0 km/h. D) 4.0 mph. E) 0.40 m/s.
A prismatic bar (diameter d0 5 18 mm) is loaded by force P1. A stepped bar (diameters d1 5 20 mm and d2 5 25 mm with radius of fillets R 5 2 mm) is loaded by force P2. The allowable axial stress in the material is 75 MPa. The ratio P1/P2 of the maximum permissible loads that can be applied to the bars, considering stress concentration effects in the stepped bar, is:
(A) 0.9
(B) 1.2
(C) 1.4
(D) 2.1