In the Ptolemaic model, Mercury and Venus had to be treated differently from the rest of the planets: Their epicycles had to remain centered on the Earth-Sun line
Indicate whether the statement is true or false
True
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A fastener bolt for the engine on a supersonic jet aircraft has a crosssectional area of 2.0 cm 2 and a length of 4.0 cm. The bolt part must be able withstand a tensile force of 10 × 104 N, and as a factor of safety this tensile force can produce a stress of no more than 50 percent of the tensile yield stress in the bolt. For sufficient resistance to fracture this critical bolt must have at least 15 percent strain to fracture. Because weight is critical for this jet aircraft design the maximum weight possible for this bolt is 40 g. The bolt is subject to temperatures up to 400°C (752°F).
To minimize the possibility of high temperature plastic strain the selected material should not be subject to more than 1/3 of its melting temperature. Of the materials covered in this chapter what would be the best material to select? Because this is a supersonic jet aircraft high priced materials can be utilized.
According to Edwin P. Hubble's discovery in 1929, the sizes of galaxy redshifts are:
A) proportional to the number of stars in the galaxy. B) inversely proportional to galaxy distances. C) inversely proportional to the number of stars in the galaxy. D) proportional to galaxy distances.
Semiconductors: The energy gap between the valence and conduction bands in a certain semiconductor is 1.25 eV. What is the threshold wavelength for optical absorption in this substance? (c = 3.00 × 108 m/s, h = 6.626 × 10-34 J ? s)
A. 599 nm B. 959 nm C. 873 nm D. 994 nm
Why can we never create energy?
A) because it is impossible to create the fundamental particles such as electrons and protons B) because energy is always conserved C) because of the equivalence of mass and energy D) because matter is always conserved E) Actually, we do create energy frequently in the lab.