Marie pushes horizontally on her desk, but not hard enough to get it moving. The friction force acting on the desk is
A) zero.
B) equal to the weight of the desk.
C) equal and opposite to her push.
D) none of the above
Answer: C
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How can we tell that some stars are relatively close to us in the sky?
a. Some stars are occasionally eclipsed by the Moon, so they must be nearby. b. Some stars vary in brightness caused by sunspots we can see because they are so close. c. Some stars appear to be extremely bright and must therefore be very close to us. d. Some stars appear to move periodically back and forth against the background stars because of Earth's movement around the Sun.
When the equation Al + O2? Al2O3 is balanced, the coefficient of Al2O3 is
a. 1 b. 2 c. 3 d. 4
A person looking through eyeglasses sees __________ images
Fill in the blank(s) with correct word
What will the temperature of the cosmic microwave background be when the average distances between galaxies are twice as large as they are today?
A. 1.365 B. 4.860 C. 5.250 D. 3.375