Download Gravity Lecture Day 2 33 Gravity Day 2

2/25/2014
Gravity?
Is gravity real or does the Earth
just suck?
Distribution of matter
Simulated image that shows the distribution of matter in the
.
Universe. Image credit: MPG
Distribution of Galaxies Survey
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Gravity Review
Gravity is a FORCE
1. What happened when you decreased the
distance between objects in the simulation?
2. What happened when you changed the mass
of the objects in the simulation?
3. What two factors affect the amount of
attraction between two objects?
4. Do you think that the force of gravity
increases with the distance from the Earth’s
surface?
• Gravity is a natural force of attraction by
ANY and ALL objects that have mass,
which is directly proportional to the mass
and inversely proportional to the
distance between them.
• When gravity pulls something towards
the Earth - we call this force the weight.
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The Law of Universal Gravitation:
• All objects in the universe attract each other
through gravitational forces. The size of the
force depends on the masses of the objects
and the distance between these objects
The Equation
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∝
•
•
•
•
•
2 Rules for Law of Universal
Gravitation:
1. The Gravitational
Force INCREASES
as the mass of the
object INCREASES
2. The Gravitational
force DECREASES
as distance
between the
objects INCREASE
Mass & Distance Affect Gravity
Directly Proportional to Mass
Inversely Proportional to Distance
F = gravitational force
∝ = proportional
m1 = the mass of one object
m2 is the mass of the other object
d is the distance between the two objects.
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Example
• ∝
• ∝
Example
• If m1 is doubled to 4kg,
• If m1 = 2 kg, m2 = 2 kg,
m2 = 2 kg, and R = 2 m
and R = 2 m
• then F is proportional to • then F is proportional to
4 x 2 / 22 = 8/4 = 2
2 x 2 / 22 = 4/4 = 1.
• 2 times higher
(since gravity is directly
proportional to mass).
• ∝
• Using the original masses,
if R is doubled to 4,
then F is proportional to 2
x 2 /42 = 4/16 = ¼
• Is this relationship similar
to what they saw in the
simulations in the lab?
• 1/4 the original force
(since gravity is inversely
proportional to the
distance squared).
Force of Gravity Demo!
Force of Gravity Demo!
1. Does the rubber band pull with the same
force on both balls?
2. What will happen to the balls when I release
the rubber band?
3. Observe both balls during the demo. What
happened?
4. Why didn’t the steel ball move as much as
the tennis ball?
The rubber band is like the gravitational force
acting on objects.
1. Is the gravitational force felt by both balls the
same?
2. Are the effects of the gravitational force the
same on both balls? Did this happen during
your simulations?
Post-Lab Questions
The Law of Universal Gravitation:
• Use your lab to answer the questions on the
Post-Lab Questions Worksheet
• All objects in the universe attract each other
through gravitational forces. The size of the
force depends on the masses of the objects
and the distance between these objects
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Gravity and mass
Gravitational force is
Gravitational force is SMALL
between objects that have a
small mass
LARGE
between objects that
have a large mass
Misconceptions about Gravity
• Mass and weight are the same quantity of
measurement
• Gravity is related to movement, closeness to earth,
or magnetic fields
• The Moon has no gravity (It does)
• Astronauts are weightless because there is no gravity
in space
• Planets with thin atmospheres have little gravity
• Planets distant from the sun have less gravity
• Gravity is stronger between the most distant objects
Gravity down under
More Misconceptions about Gravity
•
•
•
•
• People
do not
fall off
the Earth
•
•
•
•
Weightlessness means no gravity.
Heavier objects fall faster than lighter ones.
Gravity is a very strong force.
There is no gravity in space, on the moon, or in a
vacuum
Gravity is caused by Earth’s spin, air pressure, push
from above.
Gravity is weaker under water.
Gravity is stronger the higher you are in a building or
mountain that’s why it is easier to fall.
Gravity will be stronger the longer and object falls.
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Two objects
dropped at
the same
time…
On the Moon – Apollo 15
A heavy geological hammer and a
falcon feather were dropped from
the same height.
They hit the lunar surface at the
same time.
Will hit the
ground at
the same
time!
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astronaut David Scott
NASA pictures
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More Misconceptions about Gravity
• Shape affects gravity
• Gravity is weaker the higher you go. That’s why things fall
faster the farther they fall.
• Astronauts are weightless because they are beyond Earth’s
gravity.
• Gravity is the attraction of the larger body. The Earth and me
are not attracted by other objects pulling up.
• Gravity causes objects to fall down through the center of the
Earth and out the other side if there was a hole in through the
Earth.
• Gravity pulls objects towards the south pole and if the Earth
would not stop it they would go off into space away from the
south pole.
• Gases are not affected by gravity.
No Gravity?
•
•
•
•
Would an apple fall?
Would things stay on earth?
What would happen?
Would the universe be the way it is today?
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Launching a satellite
Weightless?
• or are they?
The cannon
ball must
move very
quickly to
orbit the
earth.
The Moon
orbits the
Earth
(NASA pictures)
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apparently weightless
GRAVITY
(pictures or animation only –not a demonstration)
• Things with mass are attracted to one another
• Things fall
• Things have weight
• Things stay in place
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Force of Gravity
Thinking about Gravity
1. Do you feel the gravitational pull from the
Earth?
2. Does the Earth experience the gravitational
pull from you?
3. Is the gravitational force between you and
the Earth the same? (Think about the rubber
band demo)
4. Does the Earth and you experience the same
effects of the gravitational force? Why?
• Can you think of a weigh to measure the
strength of gravity at the surface of the Earth?
Strength of Gravity on Earth
Strength of Gravity on Earth
• Rearranging the gravity equation for Earth we
get:
Strength of
gravity =
= 9.8m/s2
• G = the gravitational constant
Gravity on Other Planets
Force of Gravity on Other Planets
Planet
• Would you weigh the same on other planets
or moons? Why or why not?
Strength of Gravity
(Earth = 1)
Mercury
0.38
Venus
0.91
Earth
1.00
Mars
0.38
Jupiter
2.54
Saturn
1.06
Uranus
0.92
Neptune
1.18
Pluto
0.06
To determine your weight on another planet: Multiply your weight
by the strength of gravity on another planet
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On the Moon
Very heavy boots- Why?
Answers?
• No gravity on the Moon
• Weightless on the Moon
• To stop them floating away
• They weigh very much less than on Earth
• To stop huge bounces
• The Moon’s gravity is very much less than the Earth’s
gravity
NASA pictures
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The Earth
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The Earth
The Moon
The Moon
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Final Thoughts…
The Universe: Gravity Video!
• The force of gravity is proportional to the
mass, and is inversely proportional to the
distance squared.
• So, as mass increases, gravity increases. As
distance increases, gravity decreases
(proportional to the square of the distance).
• COMPLETE your post-lab questions and TURN
IN!
• Answer the questions on the back of your
gravity notes
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Gravity
• Sir Isaac Newton (1642-1727) developed a law of
gravitation which said all objects in the universe
attract each other through gravitational force
• Universal Gravitation Equation
•
F = G m1m2
d2
• Equation shows grav. Force ↑ as both masses ↑
• Equation shows grav. Force ↓ as the distance b/w
masses ↑
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