13 Universal Gravitation
March 21st/24th
• HW due – prep for quest
• HW assigned: Read & Study 12.1-12.3; complete #1-7 at end
of chapter
• In class today:
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DO NOW: Time to prep for quest
Reviewed rest of Ch. 9 HW
Took Quest
Ch 12 - Which of the 4 fundamental forces is the strongest?
• DEBATE:
– Students will choose Electromagnetic, gravity, strong nuclear, or weak nuclear –
choose a side of the room; can move at any time based on arguments and
rebuttals
• DEBATE:
– If gravity is so weak, why do we notice gravity between us and Earth?
– Why do we notice gravity between us and Earth, but not us and other stuff in this
room?
– Pending time, Started Ch. 12, universal gravitation 12.1-12.3 slides
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Take out your HW!
• #8 – 12
• #13 – 15
– (both at the end of CH 9)
• Rapid Popcorn review – PAY ATTENTION
• You will have another 5 minutes after this
to prep for your Open Notes Quest
13 Universal Gravitation
Quest
• Normal procedures apply – phones / mp3s
out and silent on your desk
• May use notes and a calculator
• When you are done, you can try the extra
credit… STAPLE TO YOUR QUEST
and turn both in TOGETHER!!!
• When you are done, you may start
tonight’s HW, or next class’ HW
13 Universal Gravitation
Gravity…
• Is difficult to measure in everyday life
• Is something we take for granted
• Is the one of the 4 fundamental forces :
• gravity,
• electro-magnetism,
• strong nuclear forces (which hold the nucleus
together)
• weak nuclear forces (involved with radioactive
decay  weak nuclear forces: are involved with
carbon dating, are a source of radioactivity, etc..)
13 Universal Gravitation
DEBATE!
• Which of the 4 fundamental forces is the
STRONGEST? (think, pair/share, then debate!)
• gravity,
• electro-magnetism,
• strong nuclear forces (which hold the nucleus
together)
• weak nuclear forces (involved with radioactive
decay  weak nuclear forces: are involved with
carbon dating, are a source of radioactivity, etc..)
13 Universal Gravitation
DEBATE!
• Move to your assigned part of the room and discuss. I
will be calling on people at random to defend their
choice. If at any time you want to switch sides, feel free
• FRONT RIGHT: gravity
• FRONT LEFT: electro-magnetism
• BACK, CENTER: strong nuclear forces (which hold
the nucleus together)
• BACK, CENTER: weak nuclear forces (involved with
radioactive decay  weak nuclear forces: are
involved with carbon dating, are a source of
radioactivity, etc..)
13 Universal Gravitation
DEBATE!
•
•
•
•
FRONT RIGHT: gravity
FRONT LEFT: electro-magnetism
BACK, CENTER: strong nuclear forces (which hold the nucleus together)
BACK, CENTER: weak nuclear forces (involved with radioactive decay  weak
nuclear forces: are involved with carbon dating, are a source of radioactivity, etc ..)
• It takes the mass of the ENTIRE WORLD to keep this ONE
PAPER CLIP on my desk…which I can easily overcome with
ELECTROMAGNETICS!
• It takes the mass of the ENTIRE WORLD to keep this
SHREDDED PAPER on my desk…which I can easily
overcome with ELECTROMAGNETICS!
• If nuclear forces were weaker than gravity, then you wouldn’t
have a chair to sit on…matter of fact, you wouldn’t exist!
Gravity would literally RIP APART EVERYTHING!
13 Universal Gravitation
DEBATE!
1. strong nuclear forces (which hold the nucleus together)
2. electro-magnetism
3. weak nuclear forces (involved with radioactive decay 
weak nuclear forces: are involved with carbon dating, are
a source of radioactivity, etc..)
4. gravity
• It takes the mass of the ENTIRE WORLD to keep this ONE PAPER CLIP
on my desk…which I can easily overcome with ELECTROMAGNETICS!
• It takes the mass of the ENTIRE WORLD to keep this SHREDDED
PAPER on my desk…which I can easily overcome with
ELECTROMAGNETICS!
• If nuclear forces were weaker than gravity, then you wouldn’t have a chair
to sit on…matter of fact, you wouldn’t exist! Gravity would literally RIP
APART EVERYTHING!
13 Universal Gravitation
DEBATE!
1.
2.
3.
4.
strong nuclear forces (which hold the nucleus together)
electro-magnetism
weak nuclear forces (involved with radioactive decay  weak nuclear
forces: are involved with carbon dating, are a source of radioactivity, etc..)
Gravity
• If gravity is SO WEAK… then WHY do we notice it
acting on us, everyday? Discuss in your groups – I
will randomly select point people to defend
• If gravity is exerted by EVERYTHING in the universe
with MASS… then WHY do we notice the gravity
earth exerts on us, but not the gravity we exert on all
the other stuff in this room?
13 Universal Gravitation
Everything pulls on
everything else.
13 Universal Gravitation
Gravity was not
discovered by Isaac
Newton.
What Newton
discovered, (legend
holds it was prompted
by a falling apple), was
that gravity is a
universal force—that it
is not unique to Earth,
as others of his time
assumed.
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12.1 The Falling Apple
MAIN CONCEPT #1
Newton reasoned that the moon is falling
toward Earth for the same reason an apple
falls from a tree—they are both pulled by
Earth’s gravity.
13 Universal Gravitation
12.1 The Falling Apple
Newton understood the concept of inertia
developed earlier by Galileo.
• He knew that without an outside force,
moving objects continue to move at constant
speed in a straight line.
• He knew that if an object undergoes a
change in speed or direction, then a force is
responsible.
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12.1 The Falling Apple
According to legend, Newton
confirmed his thoughts about
gravity while sitting under an
apple tree.
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12.1 The Falling Apple
Newton saw the apple fall, or maybe even felt it fall on his
head. Perhaps he looked up through the apple tree
branches and noticed the moon.
• He may have been puzzled by the fact that the moon
does not follow a straight-line path, but instead
circles about Earth.
• He knew that circular motion is accelerated motion,
which requires a force.
• Newton had the insight to see that the moon is
falling toward Earth, just as the apple is.
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12.2 The Falling Moon
MAIN CONCEPT #2
The moon is actually falling toward
Earth but has great enough tangential
velocity to avoid hitting Earth.
13 Universal Gravitation
12.2 The Falling Moon
• Newton realized that if the moon did not fall, it
would move off in a straight line and leave its
orbit.
• His idea was that the moon must be falling
around Earth.
• Thus the moon falls in the sense that it falls
beneath the straight line it would follow if no force
acted on it.
• He hypothesized that the moon was simply a
projectile circling Earth under the attraction of
gravity.
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12.2 The Falling Moon
If the moon did not fall, it would follow a straight-line path.
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Newton
compared motion of
the moon to a cannonball fired
from the top of a high
mountain.
• If a cannonball were fired
with a small horizontal
speed, it would follow a
parabolic path and soon hit
Earth below.
• Fired faster, its path would
be less curved and it would
hit Earth farther away.
• If the cannonball were fired
fast enough, its path would
become a circle and the
cannonball would circle
indefinitely.
His Original Drawing!
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12.2 The Falling Moon
Both the orbiting cannonball and the moon have a
component of velocity parallel to Earth’s surface.
This sideways or tangential velocity is sufficient to ensure
nearly circular motion around Earth rather than into it.
With no resistance to reduce its speed, the moon will
continue “falling” around and around Earth indefinitely.
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12.2 The Falling Moon
Tangential velocity is the “sideways” velocity—the
component of velocity perpendicular to the pull of gravity.
13 Universal Gravitation
12.2 The Falling Moon
If the force that pulls apples
off trees also pulls the moon
into orbit, the circle of the
moon’s orbit should fall 1.4
mm below a point along the
straight line where the moon
would otherwise be one
second later.
13 Universal Gravitation
12.2 The Falling Moon
It wasn’t until after Newton invented a new branch of
mathematics, calculus, to prove his center-of-gravity
hypothesis, that he published the law of universal
gravitation.
Newton generalized his moon finding to
all objects, and stated that all objects in
the universe attract each other.
13 Universal Gravitation
12.2 The Falling Moon
Why doesn’t the moon hit Earth?
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12.3 The Falling Earth
MAIN CONCEPT #3
Newton’s theory of gravity confirmed the
Copernican theory of the solar system.
THINK BACK - - What is the “Copernican
Theory”?
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12.3 The Falling Earth
No longer was Earth considered to be the center of the
universe.
• It became clear that the planets orbit the sun in the
same way that the moon orbits Earth.
• The planets continually “fall” around the sun in
closed paths.
13 Universal Gravitation
12.3 The Falling Earth
The tangential velocity of Earth about the sun allows it to
fall around the sun rather than directly into it.
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12.3 The Falling Earth
What theory of the solar system did Newton’s
theory of gravity confirm?
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12.4 Newton’s Law of Universal Gravitation
MAIN CONCEPT #4
Newton discovered that gravity is universal.
Everything pulls on everything else in a way
that involves only mass and distance.
13 Universal Gravitation
March 25th/26th
• HW due – Ch. 12 #1-7
• HW assigned:
– Read & Study 12.4 – 12.5; complete #8-12 at end of chapter
– Finish today’s notes if necessary!
• In class today:
– We will continue Ch. 12, universal gravitation
– Pending last class’ progress, may start/complete the 12.1-12.3
Newton’s Law of Universal Gravitation slides
– Remainder of class to complete the web based Universal Gravitation
Lab
• Do Now:
– Close Read – directions to be announced!
– TAKE BOTH PAPERS on my desk
13 Universal Gravitation
Francis Thompson
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Was an English poet and ascetic (abstained from EVERYTHING including
food). (father was a doctor = wealthy)
Went to University and Medical School, but took no real interest in his
studies and never practiced as a doctor, moving instead to London in 1885
to try to become a writer. No luck; had to sell matches and newspapers for
a living.
During this time, he became addicted to opium, and lived on the streets
In 1888 he was 'discovered' after he sent poetry to the magazine Merrie
England. He was sought out by the editors (husband & wife) and rescued
from the verge of starvation and self-destruction. The couple gave him a
home and arranged for publication of his first book, Poems in 1893.
Critics loved him, and Tolkien (Lord of the Rings author) was a fan!
But…He remained ill for the rest of his life. (extreme poverty, ill-health, and
an addiction to opium took a heavy toll on Thompson, even though he found
success in his last years.)
He eventually died from tuberculosis, at the age of 48.
13 Universal Gravitation
DO NOW - Close Reading Steps
• While you are
completing these
steps, your ultimate
goal should be to
consider the
following question:
• How does this poem
relate to Newton’s
Law of Universal
Gravitation? Cite
specific evidence!
AllGravitation
things
13 Universal
by immortal power
near or far
to each other
hiddenly linked are.
That thou cans't not stir a flower
without troubling a star.
• How does this poem relate to Newton’s Law of Universal
Gravitation? Cite specific evidence!
13 Universal Gravitation
13 Universal Gravitation
Your Task…
• Discover Newton’s Law of Universal
Gravitation!
• Using a PHET simulation, you will explore
Newton’s Law of Universal Gravitation.
(Newton did this WITHOUT computers….you
can definitely do this easily with
technology!!!)
• Finish the Simulation Worksheet up
to #17 – if not, it is HOMEWORK!