Explain why you don

1
Universal Gravitation

• Explain why you dont feel pulled to the stars in
  the sky even though they have gravity.

2
Gravity
Notes
What we know Gravity pulls all things down and
pulls everything at the same rate of 9.8
m/s2. What we dont knowHow gravity works if
were not on Earth.
3
Gravity
Lets pretend we live in a Universe where this
one asteroid is the only thing in existence. No
people, no stars, no dust, nothing except this
asteroid. What can we say about gravity?
4
Gravity
Since gravity is a force, it has to be between
two things.
5
Gravity
Since gravity is a force, it has to be between
two things. These weights want to pull
together because of gravity. What if we replace
the left one with a 20 lb weight?
6
Gravity
Since gravity is a force, it has to be between
two things. These weights want to pull
together because of gravity. What if we replace
the left one with a 20 lb weight? The force
pulling them together will double.
7
Increase mass of objects
Increase force of gravity between objects
directly proportional
8
Gravity
What if we keep both of them at the same
mass, but double the distance?
9
Gravity
What if we keep both of them at the same
mass, but double the distance? The force
between them is cut down to ¼.
10
Increase distance between objects
Decrease force of gravity between objects by
alot! Inversly proportional
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Equations
Equations

• Universal Gravitation

more mass F G more distance
F The force of gravity in m/s2 m Masses in
kg. More mass means stronger
gravity! r The distance between the objects
in meters. More distance means a lot less
gravity! G gravity constant 6.674
x 10-11 N x m2/kg2 Used to calculate the
force of gravity between two objects
m1 m2 r2
12
Gravity
Notes
Gravity is what keeps the Earth going around the
sun. Imagine we could take the sun and squeeze it
down into a tiny ball like this without changing
anything else What would happen to the
Earth?
13
Gravity
Gravity is what keeps the Earth going around the
sun. Imagine we could take the sun and squeeze it
down into a tiny ball like this without changing
anything else What would happen to the
Earth? Nothing! The distance from the middle of
the sun to the middle of the earth didnt change,
so gravity is the same!
14
Gravity
What if you were standing on the Earth and the
Earth began to shrink? Would you feel gravity
change?
15
Gravity
What if you were standing on the Earth and the
Earth began to shrink? Would you feel gravity
change? Yes! The distance between you and
the middle of Earth got smaller and mass didnt
change, so gravity goes way up!
16
Gravity
Lastly, we can use Newtons second law to link
mass and weight. F m a Force (weight) mass
of person acceleration of gravity
17
Gravity
Lastly, we can use Newtons second law to link
mass and weight. F m a Force (weight) mass
of person acceleration of gravity The moon has
only about 1/6 the acceleration of gravity Earth
has. What does this do to your weight?
18
Gravity
Lastly, we can use Newtons second law to link
mass and weight. F m a Force (weight) mass
of person acceleration of gravity The moon has
only about 1/6 the acceleration of gravity Earth
has. What does this do to your weight? Divides
it by 6. If a goes down, F goes down!
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Gravity practice
classwork
1) 2) If the earth somehow grew larger
without gaining in mass, what would happen to
your weight? What if the earth got smaller? 3)
Jupiter has a mass more than 300 times that of
earth, but the acceleration of gravity there is
only 3 times more than on earth. Whats going on
here?
?
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Calculating gravity

1. Calculate the force of gravity in a 1kg mass at
   the Earths surface. The mass of the Earth is 6
   x 1024 kg and its radius is 6.4 x 106 m.
2. Calculate the force of gravity on the same 1kg
   mass if it were 6.4 x 106 m above the Earths
   surface. (two of Earths radii from the Earths
   center)
3. Calculate the force of gravity between Earth and
   the moon (mass 7.4 x 1022 kg). (average Earth
   moon distance is 3.8 x 108m)
4. How about the Earth and the Sun? (Suns mass
   2.0 x 1030 kg and average Earth Sun distance
   1.5 x 1011 m )
5. Calculate the force of Gravity between a newborn
   baby (mass 3kg) and the planet Mars (mass 6.4
   x 1023 kg ) when Mars is at its closest distance
   to the Earth. (5.6 x 1010 m )
6. Calculate the force of gravity between the
   newborn baby and the obstetrician with a mass of
   100kg when he/she is .5m from the baby. Who
   exerts more gravitational force on the baby, Mars
   or the doctor? By how much?

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Exit Question 20

• 4.14a
• What happens to the pull of gravity as your
  distance from the Earth increases?
• a. It gets less and eventually goes away
• b. It stays the same no matter how far
• c. It gets less but never totally goes away
  
• d. It suddenly stops right where the air does
• e. As soon as you're off the Earth, there is no
  gravity
• f. It gets weaker and fades where the air ends