What do most astronomers now think explains why active galaxies are so energetic?
A. intense magnetic fields
B. a supernova explosion
C. an immense black hole
D. None of these choices is correct.
Answer: C
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Carbon-fiber reinforced epoxy is being used as the outer surface material in high-performance aircraft where the surface temperatures are elevated because of the friction with air. The maximum-use temperature for epoxy resin is approximately 260°C. A composite sheet is produced from 75 vol % carbon fiber and 25 vol % epoxy resin. Assume that the epoxy and carbon are stress free at room temperature, and the composite is heated from room temperature (22°C) to 260°C during service. The elastic modulus of the carbon fiber is 380 GPa, the ultimate tensile strength is 2.4 GPa, and the axial thermal expansion coefficient is –0.7×10-6 K-1 . The elastic modulus of the epoxy resin is 4 GPa, the ultimate tensile strength is 0.07 GPa, and the thermal expansion coefficient is 60×10-6 K-1 .
(a) Calculate the thermal stresses, both the magnitude and sign, in the epoxy matrix and in the carbon fibers at 260°C. Assume that each material is elastic at all temperatures, and that there is good interface bonding. (b) Calculate the composite strain in heating from 22°C to 260°C. (c) Is there any component of this design that might cause a problem? Comment on the results in comparison to the data provided. If data is not available on these components, justify your evaluation of this design based upon your knowledge of these materials.
Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into the galaxy's disk?
A) WIMPs are lightweight enough that they have expanded out into the halo. B) WIMPs were produced by stars in the early stages of galaxy evolution and therefore are only found in globular clusters in the halo. C) WIMPS rarely interact with other particles and therefore cannot settle into galactic disks. D) Shock waves from generations of supernovae have blown the WIMPs out into the halo. E) WIMPs annihilate when they come into contact with ordinary matter, such as stars.
The Sun is a ball of gas that is on fire
Indicate whether the statement is true or false
A free electron and a free proton are released in identical electric fields. Compare the magnitudes of their accelerations
1.It is millions of times greater for the electron. 2.It is thousands of times greater for the electron. 3.They are equal. 4.It is thousands of times smaller for the electron. 5.It is millions of times smaller for the electron.