Briefly explain why we think white-dwarf supernovae are useful for measuring cosmic distances
What will be an ideal response?
They all come from explosions of white dwarfs that reach the white-dwarf limit, so we expect them all to have the same luminosity; observations of white-dwarf supernovae for which we can measure distance independently confirm that they all have the same luminosity. Since we can assume we know their luminosity, we can use their apparent brightnesses to determine distance from the luminosity-distance formula.
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In a region of space where the electric field is radial, suppose that Er = C/r3, where C is a positive constant. Is the charge density ? in this region positive, negative, or zero? (Note: ? might depend on r in this region.)
A. Positive B. Negative C. Zero D. This is impossible.
The nichrome heating element in an electric drier operates on 240 V and generates heat at the rate of 2.0 kW
The heating element shorts out and, in repairing it, the repairman shortens the nichrome wire by 10%. (Assume the temperature is unchanged. In reality, the resistivity of the wire will depend on its temperature.) What effect will the repair have on the power dissipated in the heating element? A) The power increases to 2.2 kW. B) The power is still 2.0 kW. C) The power decreases to 1.8 kW. D) None of the given answers is correct.
The resultant force R (in newtons) for the force system shown in the figure is:
(A) 29.5i 1 36.4j – 81.4k
(B) 229.5i – 36.4j 1 78.3k
(C) 236.4i 1 29.5j 1 81.4k
(D) 239.3i 1 36.4j 1 122.2k
A 1.33 kg dense mass is on the end of a 4.50 m "string"
(a) What would be the period of the pendulum on Earth? (b) If the mass were doubled, what would be the frequency of oscillation? (c) What would be the period on the planet Mars where gravity is 38.% that of the Earth?