PHY100H5 Chapter Notes - Chapter 3: Aristotelian Physics, Classical Mechanics
Chapter 3 How Things Move Galileo Asks the Right Questions
3.1 Aristotelian Physics: A Common Sense View
- Aristotle’s physics was based on natural motions that were either upward or downward, while horizontal motion
seemed as if it needed activity to maintain the motion
- He believed these pushes and pulls were needed because the objects must be forced to behave, as opposed
to following their own natural motion (these were “violent motions”)
- Believed that the moon, sun, planets, and stars were made of a pure substance not found on Earth (ether)
3.2 How Do We Know? Difficulties with Aristotelian Physics
- According to Aristotle’s physics, heavier objects should drop faster than lighter ones, but today’s theories
predict that for two objects of the same shape, the lighter one will fall a little slower due to air resistance
- air resistance is the resistance to the motion of an object through the air die to the object’s collisions with
numerous air molecules
- testable by letting two objects fall in a vacuum (like a rock and a feather) with no air resistance; they will fall
at the same speed
- Galileo’s Law of Falling: if air resistance is negligible, then any two objects that are dropped together will fall
together, regardless of their weights and shaped, and regardless of the
substances of which they are made
- Friction is the slowing of objects in motion due to the roughness of the object on a
surface
- Galileo concluded that in absence of friction, a ball that once started rolling on a
horizontal surface would roll forever (if there were no friction, a ball would speed
up on any downward incline and would slow down on any upward incline)
- The scientific process is the dynamic interplay between experience (experiments
and observations) and thought (creatively constructed theories and hypotheses)
3.3 The Law of Inertia: The Foundation of Newtonian Physics
- An objects inertia is its tendency to maintain a state of motion, whether moving or
remaining at rest (is simply a word that stands for the unexplainable fact that unassisted
objects do keep moving)
- Law of Inertia: A body that is subject to no external influences (external forces) will stay at
rest if it were at rest to begin with and will keep mobbing if it was moving to begin with;
in the latter case, its motion will be a straight line at an unchanging speed. All bodies have inertia.
- Outer space refers to those regions of the universe outside of Earth (including the atmosphere) and outside of
other astronomical objects
3.4 Measuring Motion: Speed and Velocity
- Average speed is the speed you would have to maintain in order to make a
trip at an unchanging speed
- Instantaneous speed is like average speed, but over an interval so small that
it can be considered the speed at that instance
- Average speed = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑
𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 which can be simplified into s = 𝑑
𝑡
- Velocity will represent speed and direction (a change in velocity results
whenever there is a change in speed and/or direction)
3.5 Measuring Motion: Acceleration
Document Summary
Chapter 3 how things move galileo asks the right questions. A(cid:396)istotle"s physi(cid:272)s was based on natural motions that were either upward or downward, while horizontal motion seemed as if it needed activity to maintain the motion. He believed these pushes and pulls were needed because the objects must be forced to behave, as opposed to follo(cid:449)i(cid:374)g thei(cid:396) o(cid:449)(cid:374) (cid:374)atu(cid:396)al (cid:373)otio(cid:374) (cid:894)these (cid:449)e(cid:396)e (cid:862)(cid:448)iole(cid:374)t (cid:373)otio(cid:374)s(cid:863)(cid:895) Believed that the moon, sun, planets, and stars were made of a pure substance not found on earth (ether) Galileo"s la(cid:449) of falli(cid:374)g: if air resistance is negligible, then any two objects that are dropped together will fall together, regardless of their weights and shaped, and regardless of the substances of which they are made. Friction is the slowing of objects in motion due to the roughness of the object on a surface. The scientific process is the dynamic interplay between experience (experiments and observations) and thought (creatively constructed theories and hypotheses)
