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BYU - PHY S 100 - Class Notes - Week 2

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BYU - PHY S 100 - Class Notes - Week 2

School: Brigham Young University
Department: OTHER
Course: Physical Science
Professor: Patricia Ackroyd
Term: Spring 2017
Tags: gravity and Physics
Name: Week 2 Notes
Description: Gravitational Interaction, including acceleration: notes based off textbook and class discussions (Sept. 11th)
Uploaded: 09/19/2017
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background image Chapter 3: Gravitational Interaction  Falling Objects  •  Isaac Newton discovered law concerning gravity's strength and how it changes with mass and 
distance 
•  Object off a cliff accelerates down (speed increasing)  Without air friction, it falls with a constant acceleration: 9.8 m/sec/sec  Rate of acceleration: g  ▪  G: Symbol representing the acceleration caused by gravity (22 mi/hr/sec or 9.8 
m/sec/sec) 
•  If there is no friction, the speed increases at a constant acceleration    
Uniform acceleration 
•  Remember F = m•a  •  In this case, a is now g  Acceleration caused by gravity   •  For a rock falling off a cliff, m and a are constant, and therefore so is F  •  Acceleration and force of gravity are constant for any given object as it falls 
*Figure 3.2 AWESOME for visuals* 
  
•  What about horizontal throws?  Curved path: acceleration at vertical speed + horizontal motion = curve downward   If I throw a ball at 15 m/sec, horizontal motion remains 15 m/sec, but you're subtracting 
from that motion the increasing downward speed from constant acceleration  
•  Fun fact!  Because downward acceleration is the same, a ball throw will hit the ground the SAME  TIME as a ball dropped straight down  Initial velocities different, but accelerations and way velocities change are same (…so curved 
one gets a head start) 
   Weight and Acceleration  •  Larger mass=harder to accelerate  •  Both accelerate at the same rate, though. So what happens? The force of gravity on the larger 
mass is greater than the force of gravity on the smaller mass! 
•  W =m • g (g is constant)  Once again, g is the acceleration caused by gravity. It is NOT gravity itself. What the heck.   So the weights of the larger and smaller objects are different; the heavier object has to be 
"pushed" harder by gravity to have the same acceleration rate as the lighter object 
  
  
Circular Motion and the Moon's Orbit  •  Newton used math to show how Earth's gravity was able to pull in the Moon and hold us on the 
earth 
Earth's force on moon is perpendicular to moon's velocity (like a ball on a string)  Force of gravity diminishes with distance   ▪  1/d^2 (d is distance between centers of two objects) 
background image   
Gravity and the Third Law of Motion 
•  Fun fact! Because moon is also pulling on the earth, Earth experiences a little acceleration too. It 
wobbles in and out of orbit around the sun, toward the moon 
•  The smaller the mass of a planet, the smaller the gravitation attraction it creates on other planets     Law of Universal Gravitation  •  The strength of the attractive force, F, between objects in space varies with the masses, M and m, 
of the two objects and the distance, d, between their centers 
F = (GmM)/d^2  G is the gravitational constant; relates the strength of the force to the masses being 
attracted and their distance apart 
•  Newton didn't know the mass of the earth (M) or G (required M to figure out); Henry Gavendish 
discovered G and M through his experiments in the late 1700s 
  
   Acceleration  •  Back to the rock and the cliff analogy: The force of gravity is stronger at the base of the cliff than 
at the top (two objects: rock and earth), but the effect is slight 
•  So, we still treat g as a constant     Gravity and Curved Space-Time  •  Law of gravity helps us find position of planets, times of eclipses, etc.  •  But what about empty space? How can a force transmit across?   •  1915: Einstein's General Theory of Relativity   Objects in empty space without unbalanced forces go straight without acceleration (straight 
through space and time) 
Planets affect the space-time around them, causing space-time to "curve"  When moon moves, it follows straightest possible path through carved space-time (an orbit)    

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School: Brigham Young University
Department: OTHER
Course: Physical Science
Professor: Patricia Ackroyd
Term: Spring 2017
Tags: gravity and Physics
Name: Week 2 Notes
Description: Gravitational Interaction, including acceleration: notes based off textbook and class discussions (Sept. 11th)
Uploaded: 09/19/2017
3 Pages 8 Views 6 Unlocks
  • Better Grades Guarantee
  • 24/7 Homework help
  • Notes, Study Guides, Flashcards + More!
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