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Lecture 2

PHY 101 Lecture Notes - Lecture 2: Mechanical Equilibrium, Net Force

Course Code
PHY 101

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Chapter 2 Notes
Aristotle on Motion
o Aristotle divided motion into two main classes
Natural motion
Proceeds from the "nature" of an object, dependent on the combination of
the four elements
Motion straight up or straight down from an object naturally falling (due to
gravity) or rising (smoke)
Violent motion
Resulted from pushing or pulling forces
Wind motion
Water motion
Motion that is caused by an external forces
When objects move not by their nature
Has difficulties because the pushes and pulls responsible for motion are not
always evident
o Quintessence
Aristotle believed that different rules apply to the heavens and asserted that
celestial bodies are perfect spheres made of a perfect and unchanging substance
which he called quintessence
o Came up with a theory that heavier objects fall faster
o Formulated his theory of the moving Earth
o Reasoned that the simplest way to account for the observed motions of the sun, moon,
and other planets through the sky was to assume that Earth and the other planets circle
around the sun
Galileo's Experiments
o Gave credence to the Copernican view of a moving Earth
o Discredited the Aristotelian ideas about motion
o Proved that an object that is twice as heavy as another will fall at the same speed with
his experiment of dropping objects off of the Leaning Tower of Pisa.
Slight difference due to the different air resistances on the different objects
o Inclined planes
Ball rolling down a steeper hill picks up more speed
Ball rolling up a steeper hill decreases speed
No slope makes the speed of the ball remain constant
Friction is what slows the ball down to a stop
In the absence of friction, if the ball is rolling on a horizontal surface, it will
continue moving at the constant speed forever, and not slow down
o The property of an object to resist changes
o A body in motion wants to stay in motion
o A body at rest wants to remain at rest
o Is not a force; it's a property of all matter to resist changes in motion
Newton's First Law of Motion
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