Download File

Gravity
It’s a phenomenon, it’s a force, it’s the
fabric of space and time …
PART 1-A: A Historical
Look at Gravity
I CAN explain how our
understanding of gravity evolved
throughout early history. This
means I can explain how the
work of men like Aristotle,
Galileo, and Copernicus built the
foundation for what we now
know about the force of gravity.
What do you already know about
gravity?
• With your seat or elbow partner divide a scrap sheet of
paper into 3 sections and
• write down the following:
1. What do you know about gravity that are facts?
2. What do you think you know but are a little fuzzy about whether it is fact
or not?
3. What don’t you understand at all about gravity?
• When you and your partner have come to a consensus,
record your statements neatly onto post-it notes
• Be ready to share out with the class
in 15 minutes.
Historically what we know about
gravity began with our ancestor’s
fascination with the heavens …
To understand where we’re going in
science, we often have to know where
we’ve been. A look back activity …
• You are going to be placed in teams of 2, 3 or 4.
• Each of you will read a short article on a historical figure in
science who impacted our understanding of gravity.
• While you read, underline places in the text that relates to
gravity. It may not explicitly say “gravity” but the
phenomenon must be directly or indirectly related to
gravity. In other words, the scientist may not have known
that what they were making scientific statements about
was gravity.
• Once you’ve read your article, share out with your partners.
• You have 10 minutes.
Making meaning of what you
have read with your partners…
• With your partners, share
your notes that are
directly related to your
scientist’s contribution to
our understanding of
gravity. (even if they did not know
it was gravity)
• Rewrite these notes on a
scrape sheet of paper
together in your own
words. Summarize long
sections when necessary.
• You have 12-15 minutes.
QUICK WRITE
DEBRIEFING
READ over the notes that you have made on
your scientist. Using a graphic organizer,
compare how your scientist understood gravity
back then with how you said on your post-its
that you understand gravity now. What are the
similarities and what are the differences?
Gravity
It’s a phenomenon, it’s a force, it’s the
fabric of space and time …
PART 1-B: A Historical
Look at Gravity
SHARING OUR UNDERSTANING OF
GRAVITY THROUGH THE DISCOVERIES
AND CONTRIBUTIONS OF DIFFERENT
SCIENTISTS
I CAN explain how our
understanding of gravity evolved
throughout early history. This
means I can explain how the
work of men like Aristotle,
Galileo, and Copernicus built the
foundation for what we now
know about the force of gravity.
Let’s revisit our “quick
write” from yesterday …
Let’s share out and see if we
share some of the same
ideas with our scientists and
if some of our own
understanding surpasses that
of our early scientists.
verses
Now you’re ready to share what you
have learned in your article with
your fellow students
• Along with your partners,
create a “mini” poster of your
findings directly related to your
scientist’s contribution to our
understanding of gravity. (even
if they did not know it was gravity)
• Remember, the findings you
put on your poster should be
in your own words!
• Be ready to share out with the
class by readying your poster
for a gallery walk.
• You have 10 minutes.
During your gallery walk make notes on each of the
scientists and their contributions to our
understanding of gravity.
Gravity
It’s a phenomenon, it’s a force, it’s the
fabric of space and time …
PART 1-C: A Historical
Look at Gravity
ENHANCING OUR UNDERSTANDING OF
GRAVITY THROUGH THE CREATION OF
A HISTORICAL TIMELINE
I CAN explain how our
understanding of gravity evolved
throughout early history. This
means I can create a historical
timeline explaining how the work
of men like Aristotle, Galileo, and
Copernicus built the foundation
for what we now know about the
force of gravity.
Create a Historic Timeline Comic Strip
• With your notes, create a
historic comic strip on the
scientists from your gallery
walk
• Be sure your illustrations
depict their contributions to
our understanding of gravity
• Through additional research,
add something interesting
about each of the scientists
that can add humor to your
comic
• Be sure your comic strip
forms a timeline as well
Create a Historic Timeline
• With your notes, create a historic timeline on the
scientists from your gallery walk notes.
• Be sure your timeline includes their contributions to
our understanding of gravity
• Add your pictures from your notes to symbolize their
contribution.
Let’s share our timelines!
An early history of our understanding of gravity
verses
Geocentric
Heliocentric
State of Physics- Newton and His Laws
By now the world knew:
• Bodies of different weights fall
at the same speed
• Bodies in motion did not
necessarily come to rest
• Moons could orbit different
planets
• Planets moved around the Sun in
ellipses with the Sun at one focus
(Kepler’s 1st law)
• The orbital speeds of the planets
obeyed Kepler’s 2nd and 3rd laws
But why??? Isaac Newton put it all together.
Newton’s Concepts
1) m (mass): How much stuff something contains
2) v (velocity): A body’s speed and direction
3) a (acceleration): The change in a body’s velocity
4) F (force): What is needed to change a body’s velocity
Newton’s Laws of Motion
1)
A body’s velocity will remain constant, unless acted upon by
an outside force = inertia
Newton’s Laws of Motion
1)
A body in motion will remain constant, unless acted upon by
an outside force = inertia
2)
A body’s acceleration depends on the force acting
upon it, and will be in the direction of that force.
Its resistance to acceleration depends on its mass.
In equation form, this is
F=ma
3)
For every force, there is an equal and opposite force.
Newton’s Law of Gravity
There is an attractive force between
two bodies called gravity. The force
of gravity depends on the masses of
the two bodies, and their separation
(squared); the larger the mass, the
greater the attraction; the larger the
separation, the smaller the attraction.
Basically, this means that the 2
most important factors that
affect gravity are Mass and
Distance.
G m 1 m2
F = 
r2
Example of Gravity – a Thrown Ball
When you throw a ball, there are
2 motions: horizontal & vertical.
The horizontal motion obeys
Newton’s first law (bodies in
motion will stay in motion). The
attractive force of gravity causes
the upward motion to decelerate,
and then change direction. You
see the composite of the two
behaviors.
Example of Gravity – a Thrown Ball
Example of Gravity – Weightlessness
You feel weight because of Newton’s third law. Gravity is pulling
you down, but the ground is not allowing you to fall. It must therefore
be exerting a force on you to keep you from falling. That force is the
weight that you feel.
If you were allowed
to fall, you would
not feel any weight.
So when you are in
free-fall, you feel
weightlessness.
Example of Gravity – Weightlessness
You feel weight because of Newton’s third law. Gravity is pulling
you down, but the ground is not allowing you to fall. It must therefore
be exerting a force on you to keep you from falling. That force is the
weight that you feel.
As an example, a sky diver is in
free fall towards the earth, and
therefore feels weightlessness.
gravity
Example of Gravity – Weightlessness
If an object is dropped from
rest, it will free-fall until it
reaches the Earth.
However, if the object is
given a velocity parallel to
the ground, the compromise
between Newton’s 1st law of
motion and gravity will be a
free-fall that always misses
the Earth, which is an orbit.
Because astronauts orbit the
Earth in free-fall, they feel
weightlessness.
velocity
gravity
Example of Gravity – a Planetary Orbit
Imagine a planet moving
sideways with respect to the
Sun. Newton’s first law says
that it will continue to move
sideways. But the law of
gravity says that it will also
be pulled towards the Sun.
The result is a combination
motion, in which the planet
falls towards the Sun, but
misses. This is an orbit.
Example of Gravity – a Planetary Orbit
Imagine a planet moving
sideways with respect to the
Sun. Newton’s first law says
that it will continue to move
sideways. But the law of
gravity says that it will also
be pulled towards the Sun.
The result is a combination
motion, in which the planet
falls towards the Sun, but
misses. This is an orbit.
If the Earth had been born at rest relative to the Sun, it would have
fallen immediately into the Sun.
Example of Gravity – Binary Stars
According to Newton’s third law,
just as the Sun exerts a force on the
Earth, the Earth exerts a force on the
Sun. However, since the Sun is so
much more massive, it doesn’t move
much. But if the Earth were more
massive, the Sun would move.
Thus, Kepler’s 3rd law is not quite
complete. The true law is
(M1 + M2) P2 = a3
where the orbital period (P) is in years,
the semi-major axis (a) is in A.U.s, and
the masses (M) are in solar units.
Example of Gravity – Tides
The effects of gravity do not depend
on the composition of a body, just its
mass and distance. The Moon exerts
a force on the Earth, but since the
Earth has a finite size, this force is
different from one side of the Earth to
the other. The side of the Earth near
the Moon gets pulled most, the center
of the Earth less, and the backside
least of all. Since most of the Earth is
solid, it doesn’t move much, but water
reacts to this difference. So we have
tides.
d
d
Summary
• Newton’s concepts: mass, velocity, acceleration, force
• Newton’s Laws of Motion
• inertia
• Force = mass x acceleration
• For every force, there is an equal and opposite force
• Newton’s Law of Gravity
• Gravitational force = GM1M2/r2
• Explains trajectory of projectiles, planetary motion, tides, etc.
• Corrected version of Kepler’s 3rd law: (M1+M2)P2=a3