A radioactive substance
A. has been damaged by exposure to radiation.
B. always emits photons which causes it to glow.
C. contains isotopes with unstable nuclei.
D. becomes active when bombarded with radio waves.
C. contains isotopes with unstable nuclei.
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What mode of fracture is present for a smooth tensile specimen when a small amount of plastic strain creates a crack that is of sufficient length to cause fracture?
(a) Mode I (b) Mode II (c) Mode III (d) Ductile
What produces the 21-cm line that we use to map out the Milky Way Galaxy?
A) atomic hydrogen B) ionized hydrogen C) molecular hydrogen D) carbon monoxide E) helium
Two thin lenses with 6.80-cm focal lengths at are mounted at opposite ends of a 18.8-cm long tube. An object is located 20.4 cm from one end of the tube. How far from the opposite end is the final image?
a. 8.88 cm b. 32.5 cm c. 4.54 cm d. 8.60 cm e. 20.4 cm
When you hear the horn of a car that is approaching you, the frequency that you hear is larger than that heard by a person in the car because for the sound you hear
a. wave crests are farther apart by the distance the car travels in one period. b. wave crests are closer together by the distance the car travels in one period. c. the car gets ahead of each wave crest before it emits the next one. d. the speed of sound in air is increased by the speed of the car. e. a speeding car emits more wavecrests in each period.