Summary

• Universal Law of Gravity
• Orbits
• Kepler's Laws

• Written Quiz    Timer
  
  
• Reading Quiz



• Orbits & Kepler
  1. Orbits are ellipses applet
  2. Planets sweep equal areas/time applet
  3. T ² ∝ r³
  
  
  • Newton's explanation
    Example #1
    
  • geosynchronous orbits
  • Using ratios
    Example #2
    
    
  • ULG and acceleration
  
  
• Interactive Example 12-7

Walker4e
Reading quiz The orbital speed of the Earth is greatest around January 4 and least around July 4. Therefore, the distance from the Earth to the Sun on January 4 is ____ its distance from the Sun on July 4.
A. greater than
B. less than
C. equal to
Answer















 
The mass of the earth is 5.98×10²⁴ kg. What is the orbital period of a satellite whose orbit radius is 4.23×10⁷ m?
A. 13.3 s
B. 3420 s
C. 13,800 s
D. 86,600 s
Answer















 
Two spacecraft in outer space attract each other with a force of 1×10⁶ N. What would the attractive force be if they were twice as far apart?
A. 4.0×10⁶ N
B. 2.0×10⁶ N
C. 5.0×10⁵ N
D. 2.5×10⁵ N
Answer















 



B. less than

The law of equal areas (Kepler's second law) requires that the orbit speed is greatest when the distance to the Sun is least. In the northern hemisphere we experience winter in January even though the orbit distance is least, because the seasons are caused by the tilt of the Earth's orbit axis with respect to the orbital plane, and not by the distance between the Earth and the Sun.
























 



D. 86,600 s





















 



D. 2.5×10⁵ N