2. Aristotle
The idea that force causes motion is
not a new concept. Greek philosophers
classified motion in two ways, natural
motion and violent motion. Natural
motion was straight up or straight
down. Objects would seek their natural
positions. To the Greeks natural motion
required no force.
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3. Aristotle
Violent motion was an imposed motion,
the result of a push or a pull. Violent
motion was imposed on objects.
Objects on their natural positions
cannot move by themselves. For nearly
2000 years only side to side was
believed to be caused by a force.
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4. Copernicus
Copernicus was the first to
question how things moved. The
circular motion of the planets and
stars was better explained if the
Earth moved around the Sun. He
died on the day his book was
published.
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5. Galileo
Born about 100 years after Copernicus
died was a very outspoken supporter of
Copernicus. This earned him a trial and
house arrest. While under arrest he
began to study motion. During those
years under arrest he proved that a
force was not needed to keep an object
moving. Without the presence of friction
an object can move forever.
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7. Newton
Newton was born on the same day
that Galileo died. By age 24 Newton
had formed his laws of motion.
Galileo was concerned with the how
not the why which is the reason that
they are Newton’s laws. Newton
showed that gravity was a force
because it changed the “state of
motion”.
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8. Newton’s First
Law
Every object continues in a
state of rest, or motion in a
straight line at constant speed,
unless it is compelled to
change that state by forces
exerted upon it.
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9. Inertia
Both Galileo and Newton used this
term to describe how things
resisted change in motion.
However Newton expanded the
idea that Galileo had and applied it
to motionless objects as well.
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10. Mass & Inertia
Newton was able to show that to
change the “state of motion” a force
had to be applied. He was also able to
show that how much matter or mass an
object had affected the forces required
to make it change its “state of motion”.
Mass is a measure of an object’s
INERTIA.
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11. Mass & Weight
Weight is a measure of the pull of
gravity. If something has a lot of mass
we say that it has a lot of weight. Think
about perspective though.
What is the Moon’s pull of gravity?
Does mass change on the Moon?
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12. Mass & Weight
On the moon I would weigh 1/6th what I
weigh on Earth. Does this mean that I
lost 5/6th of my body? Hopefully not! Mass
is constant and it is the measure of matter
in an object where weight is the force of
gravity on that same object. Even though
these two quantities are different they are
proportional in the same spot.
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13. Mass & Volume
Two more quantities that are often
mistaken is that the bigger an object is
the mass it contains. Which has a larger
volume a car battery or a sofa cushion?
Obviously a car battery has less volume
than a sofa cushion but the battery has
the higher mass.
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14. The newton
The SI unit of force is the newton. Since
weight is the measure of gravity and
gravity is a force. Then we can measure
the force that the Earth exerts on an
object. This unit is found by multiplying
the force of gravity by the mass of an
object. Since the SI unit for mass is the
kilogram then the newton is w=mg.
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15. Net Forces
What forces act on a book laying
motionless on a table? There are two
forces that are acting on the book. The
force of gravity and the support force.
The forces are at equilibrium, meaning in
this case the object is not moving.
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16. Force Diagram
What happens to things in the absence of
force. They stay at rest or they continue in
motion. In other words in the absence of a
net force objects do not change their state
of motion. What is a net force? We can
use vectors to explain this.
SEE BOARD
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17. Net Forces
What about a book sliding across a table?
There are three forces now. The new
force is called the normal force. Can a
moving object be at equilibrium?
YES!!!!!
But first we must look at the addition of
forces.
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18. Net Forces
Now lets look at something hanging from
two ropes. What does the force diagram
look like of a trapeze artist hanging
motionless in the air.
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19. Forces
What about when an object is released?
LETS GO BACK TO THE BOARD
Straight line
Circular
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20. The Moving Earth
When Copernicus announced that the
Earth moved, his idea was stimulated
much debate. Here is an example of the
argument used to disprove Copernicus.
Can you use what you know to refute this
argument?
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