Dark energy was discovered as astronomers studied
A) black holes.
B) the motion of galaxies within galactic clusters.
C) redshift of distant galaxies.
D) weakly interacting massive particles.
C
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In a famous experiment done at the end of the 19th century, two metal electrodes were placed in an evacuated glass tube. A high voltage was applied between them. By using an appropriate metal for the cathode (e.g., sodium or potassium),
it was found that a current could be made to flow when the cathode was illuminated with blue light, but no current would flow when red light was used, no matter how bright the light. Einstein was able to explain this strange phenomenon, and as a result, this experiment was considered the first experimental demonstration of A) the particle nature of light. B) the wave nature of electrons. C) the particle nature of electrons. D) the wave nature of light. E) the equivalency of electron and photons.
A child runs at 3.0 m/s and jumps onto a sled, initially at rest. If the child's mass is 36 kg, and if the child and sled slide off together at 2.0 m/s after the collision, the sled's mass is
A. 48 kg. B. 18 kg. C. 12 kg. D. 36 kg.
Why does the Moon not have a magnetic field?
A. The Moon's magnetic field was destroyed by bombardment by planetesimals. B. The Moon lacks an atmosphere. C. It rotates very slowly and has a cold interior. D. Any field the Moon could generate is overwhelmed by Earth's field. E. Only planets can have magnetic fields.
A +7.00 ?C point charge and -9.00 ?C point charge are placed along the x-axis at x = 0.000 cm and x = 40.0 cm, respectively
Where must a third charge, q, be placed along the x-axis so that it does not experience any net electric force due to the other two charges? A) -0.200 m B) 2.99 m C) -0.187 m D) -2.99 m E) 0.187 m