A certain source of sound waves radiates uniformly in all directions. At a distance of 20 m from the source the intensity level is 51 db. What is the total acoustic power output of the source, in watts?
(Note: The reference intensity I0 is 1.0 × 10-12 W/m2.)
6.3 × 10-4 W
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How do astronomers know that the luminous regions of active nuclei of active galaxies are very small?
a. Their nuclei are much brighter than expected. b. The orbits of stars near the center of the galactic nuclei are faster than expected. c. The luminosity of the nuclei varies over short times. d. The color of the nuclei is different than expected.
A helium balloon rises because of Archimedes' principle
a. True b. False Indicate whether the statement is true or false
An electron moves in a region where the magnetic field is uniform, has a magnitude of 60 ?T, and points in the positive x direction. At t = 0 the electron has a velocity that has an x component of 30 km/s, a y component of 40 km/s, and a z component of zero. What is the pitch of the resulting helical path?
a. 13 mm b. 32 mm c. 24 mm d. 18 mm e. 3.8 mm
Which of the following statements about scientific theories is false?
A) Theories are not taken seriously if they contradict older, more developed, theories. B) A theory is a model designed to explain a number of observed facts. C) If even a single new fact is discovered that contradicts what we expect according to a particular theory, then the theory must be revised or discarded. D) A theory must make predictions that can be checked by observation or experiment. E) A theory can never be proved beyond all doubt; we can only hope to collect more and more evidence that might support it.