Before the decoupling,
A) deuterium produced electrons and positrons.
B) protons and electrons combined to form atoms.
C) the universe was opaque to radiation.
D) there was more helium than hydrogen.
E) the universe was transparent to radiation.
C
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A locomotive is pulling 19 freight cars, each of which is loaded with the same amount of weight. The mass of each freight car (with its load) is 37,000 kg
If the train is accelerating at 0.22 m/s2 on a level track, what is the tension in the coupling between the second and third cars? (The car nearest the locomotive is counted as the first car, and friction is negligible.)
Making a laser work depends on creating a population inversion, meaning a situation in which:
a. most electrons are in the lower energy state. b. most photons are in the lower energy state. c. more electrons are in a high-energy than a low-energy state. d. the directions of electron motion are reversed, or inverted. e. more photons are in a high-energy than a low-energy state.
A small sphere attached to a light rigid rod rotates about an axis perpendicular to and fixed to the other end of the rod. Relative to the positive direction of the axis of rotation, the angular positions of the sphere are negative, its angular velocity is negative, and its angular acceleration is positive. The sphere is
a. rotating clockwise and slowing down. b. rotating counterclockwise and slowing down. c. rotating clockwise and speeding up. d. rotating counterclockwise and speeding up. e. first rotating counterclockwise and then clockwise.
About how hot is the core of Earth?
A. 15 million K B. 6,500 K C. 300 K D. 15,000 K