Universal Gravitation
Explain why you don’t feel pulled to the stars in the sky even
though they have gravity.
Gravity
Notes
What we know:
Gravity pulls all things down and pulls
everything at the same rate of 9.8 m/s2.
What we don’t know:
How gravity works if we’re not on Earth.
Gravity
Let’s 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?
Gravity
Since gravity is a force, it has to be
between two things.
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?
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.
Increase
mass of Increase force of gravity between
objects
objects
“directly proportional”
Gravity
What if we keep both of them at the
same mass, but double the distance?
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.
Increase
distance
between
objects
Decrease force of gravity between
objects by alot!
“Inversly proportional”
Equations
Universal
Gravitation
Equations
m1 • m2 more mass
F=G
r2
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
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?
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 didn’t
change, so gravity is the same!
Gravity
What if you were standing on the Earth
and the Earth began to shrink? Would
you feel gravity change?
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
didn’t change, so gravity goes way up!
Gravity
Lastly, we can use Newton’s second law
to link mass and weight.
F=m•a
Force (weight)
mass of person
acceleration of gravity
Gravity
Lastly, we can use Newton’s 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?
Gravity
Lastly, we can use Newton’s 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!
Gravity practice
1)
classwork
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. What’s going on here?
Calculating gravity
1. Calculate the force of gravity in a 1kg mass at the Earth’s surface.
The mass of the Earth is 6 x 1024 kg and it’s 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 Earth’s surface. (two of Earth’s radii from the
Earth’s 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? (Sun’s 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 it’s
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?
Exit Question #20
4.14a
What happens to the pull of gravity as your distance from
the Earth increases?
a.
b.
c.
d.
e.
f.
It gets less and eventually goes away
It stays the same no matter how far
It gets less but never totally goes away
It suddenly stops right where the air does
As soon as you're off the Earth, there is no gravity
It gets weaker and fades where the air ends