A machine gun is attached to a railroad flatcar that rolls with negligible friction. If the railroad car has a mass of 6.25 × 10^4 kg, how many bullets of mass 25 g would have to be fired at 250 m/s off the back to give the railroad car a forward velocity of 0.3 m/s?
a. 400
c. 3000
b. 2000
d. 5000
C
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If the density of the universe is greater than critical, then
A) the universe will end up as nothing but black holes. B) the universe will continue expanding forever. C) there is more matter than energy. D) the universe is closed, gravity wins, and will shrink to the Big Crunch. E) the universe is flat, and Euclid is right.
The time interval during which a nucleus has a 50% probability of decaying is its _________
Fill in the blank(s) with correct word
Which of the following best describes the modern definition of a constellation?
A) a region of the celestial sphere B) a pattern of bright stars in the sky C) a Greek mythological figure D) a collection of stars that are near one another in space E) a group of stars that all lie at about the same distance from Earth
The Sun's declination on June 21st is
A) +23.5 degrees. B) 0 degrees. C) -23.5 degrees. D) different depending on where you are on Earth.