Observationally, a key distinction between a white dwarf supernova and a massive-star supernova is that the spectrum of a massive-star supernova shows prominent hydrogen lines, while the spectrum of a white dwarf supernova does not. Why don't we see prominent hydrogen lines in the spectrum of a white dwarf supernova?
A) White dwarf supernovae occur so far away that their hydrogen lines become too dim to see.
B) During a white dwarf supernova, all the white dwarf's hydrogen fuses instantly to make helium.
C) White dwarfs are stellar corpses that contain little or no hydrogen, so they do not show hydrogen lines when they explode.
D) The hydrogen lines of white dwarf supernovae are so redshifted that we cannot detect them.
C) White dwarfs are stellar corpses that contain little or no hydrogen, so they do not show hydrogen lines when they explode.
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Equilibrium: A 3.00-m-long ladder, weighing 200 N, leans against a smooth vertical wall with its base on a horizontal rough floor, a distance of 1.00 m away from the wall. The ladder is not completely uniform, so its center of gravity is 1.20 m from its base. What force of friction must the floor exert on the base of the ladder to prevent the ladder from sliding down?
A. 93.3 N B. 130 N C. 28.3 N D. 102 N E. 150 N
Why do firemen use infrared cameras to look into smoke filled buildings?
A) The smoke itself gives off a lot of infrared light. B) They can avoid all the broken glass because glass is bright in infrared light. C) They can see through the smoke to look for people inside the room. D) They can see if there is anything radioactive in the room. E) They can tell if the smoke is toxic.
A metal rod of length L in a region of space where a constant magnetic field points into the page rotates about an axis through its center at constant angular velocity ?. The ends, A and E, make contact with a split ring that connects to an external circuit. The current in the external circuit of resistance R has magnitude
A.
B.
.C.
.D.
.E.
.Magnetic reversals of the Earth's field have occurred on the average every 200,000 years over the past 5 million years. When was the last magnetic reversal?
a. about 200,000 years ago c. about 990,000 years ago b. 1792 d. about 780,000 years ago