Why did Ptolemy have the planets orbiting Earth on "circles upon circles" in his model of the universe?
What will be an ideal response?
to explain the fact that planets sometimes appear to move westward, rather than eastward, relative to the stars in our sky
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The electron orbiting the proton in the hydrogen atom is at a distance 
m. What is the ratio of the electric force to the gravitation force between these particles? (The mass of the proton is 1.67
kg and the mass of the electron is 
a. 
b. 
c. 
d. 
e. 
The force fo static friction between two surfaces is parallel to the surface of contact, and in in the direction that opposes relative motion
Indicate whether the statement is true or false
How would you describe the volume of the following object?
the amount of water in a swimming pool A) microscopic B) macroscopic C) submicroscopic D) all of the above E) none of the above
Magnetic Field of a Long Wire: A long straight wire carrying a 4-A current is placed along the x-axis as shown in the figure. What is the magnitude of the magnetic field at a point P, located at y = 2 cm, due to the current in this wire? (?0 = 4? × 10-7 T ? m/A)
A. 20 ?T B. 30 ?T C. 40 ?T D. 50 ?T E. 60 ?T