1. Chapter 13 Universal Gravitation
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2. Newton and the Moon
Newton realized that Earth’s gravity was the centripetal force that kept the moon in orbit.
Also discovered that gravity was weaker at that great distance.
Gravity
force
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3. Gravity & Distance
We don’t notice that gravity gets weaker as we move away from Earth because we rarely go very far.
Moon is 30 Earth
diameters away
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4. Universal Law of Gravity
Force of gravity has magnitude given by
(Gravity Force) = (G) x
( Mass of Object A ) x ( Mass of Object B)
( Distance ) x ( Distance )
DISTANCE
Object A
Object B
Force
Force
Equal and opposite forces
(Newton’s Third law)
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5. Universal Gravity Constant, G
In the formula for gravity force, we have
G = N m2 / kg2
= 6.67 x 10–11 N m2 / kg2
The formula and the constant are called “universal” because, up to now, this theory predicts gravity anywhere in the universe.
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6. Cavendish Experiment
For non-astronomical objects gravity force is very small.
Need sensitive balance to detect.
First measured by Cavendish in his home laboratory in 1797.
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7. “Intelligent Falling”
"Things fall not because they are acted upon by some gravitational force, but because a higher intelligence, 'God' if you will, is pushing them down,"
Universal Law of Gravity
The Onion, August 17, 2005
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8. Sample Problem Here is an example of using the formula
(Gravity Force) = (G) x
( Mass of Object A ) x ( Mass of Object B)
( Distance ) x ( Distance )
Object A (1 kg mass)
Object B (Earth)
Force
DISTANCE = Earth’s Radius
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9. Sample Problem Find gravity force for a 1 kg mass on surface of Earth.
(Force) = (6.67 x 10–11) x
Value comes out to 9.8 Newtons (check this with your calculator; it’s good practice).
( 1 ) x ( 6 x 1024 )
( 6.38 x 106 )2
Universal Gravity Constant, G
Earth’s Radius
Earth’s Mass
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10. Sample Problem (cont.) Find gravity acceleration on a 1 kg mass.
Using Newton’s Second Law,
(Acceleration) = =
Answer is 9.8 m/s2, which we’ve been rounding off as 10 m/s2.
(Force)
(Mass)
( 9.8 N )
(1 kg )
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11. Inverse Square Law
1/4 Earth Gravity
Gravity force weakens with distance as the inverse of the square of the distance.
Geometric property of area and distance.
Earth Gravity
Outer circle is twice Earth’s radius
so it has 4 times the area
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12. Demo: Perspective These four figures are equally spaced in
At four times the distance face is 16th the size.
At twice the distance,
the face is ¼ the size.
These four figures are equally spaced in
distance and, in perspective,
you are standing that distance from the first.
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Physics 1 (Garcia) SJSU

13. Demo: Perspective Cards
Hold large card at arm’s length.
Close one eye then hold small card at a distance such that it is same size as large card.
That distance will be half way between your eye and large card. 1 2 3 4 1
Arm’s length
Half
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14. Weightlessness
In deep space, far away from all stars, planets, etc. there is almost no gravity force.
In orbit near Earth, gravity is still strong (only 10% less than on surface).
Why are Shuttle and Space Station astronauts
“weightless”?
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Earth is nearby

15. Freefall & Weightlessness
Freefall is a state of weightlessness, even though gravity is present.
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16. Movie: Roller Coaster http://www. coasterclub
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17. Check Yourself Sit in a seat on Ferris wheel. While it is rotating:
Where do you feel heaviest?
Where do you feel lightest?
What if it stops? E A B C D
Velocity
Centripetal
Force
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18. Demo: Ferris Wheel in Hand
Support
Force
Place a heavy object in your hand, hold your arm out and rotate your arm, palm upward, in a small circle.
Object will feel lightest at the top of the circle and heaviest at bottom.
Support
Force
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19. Demo: Falling Slinky Hold a Slinky and its weight stretches it out.
Now drop it.
In freefall it’s in a state of weightlessness so Slinky immediately contracts.
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20. NASA’s “Vomit Comet”
NASA has a special airplane for training astronauts in free-fall weightless conditions.
The “Vomit Comet” nickname tells you it’s quite a wild roller-coaster ride.
The plane flies between 20,000 and 30,000 feet, same as commercial flights.
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21. Flight of the “Vomit Comet”
Weightless
Freefall
At the top of the arc, the plane’s trajectory is projectile motion.
Power
Climb
Pull out
of Dive
Boeing 707 (modified)
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