In western movies you often see a person putting his ear against the rails in the train tracks to find out if a train is approaching
Why would this give him advance knowledge of the approach of a train?
The speed of sound in metals is considerably faster than the speed of sound in air.
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O3 and O2 are the two ______________ of the element oxygen
Fill in the blank(s) with correct word
During a temperature inversion _______.
What will be an ideal response?
A rope is connected to a scale on the left and exerts a force 
and a rope is connected to the same scale on the right and exerts a force 
The scale shows a reading of 
= 120 N. The values of 
and 
are
a. 
b. 
c. 
d. 
Why does the Cepheid "standard candle" have limited usefulness beyond 100 Mpc?
A) Distant galaxies are seen in the past, when Cepheid variables behaved differently than they do today. B) Distant galaxies are so young they do not contain Cepheids. C) Cepheids are too faint to be seen beyond that distance, even with HST. D) The light variability of Cepheids diminishes with distance so they do not appear to vary there. E) Older distant Cepheids are of population II, where the period-luminosity relation no longer works.