Chapter 2 Notes: Motion, Inertia, and Newton's First Law

Ptolemy: geocentric model, Earth at center; sun, Moon, planets revolve around Earth; stars fixed in a celestial sphere.
Copernicus: heliocentric model, Sun at center; planets (including Earth) orbit the Sun; initial reception was poor.
Galileo supported Copernicus; extended discussion of motion and introduced inertia concepts; his ideas faced resistance but were validated over time.

Universe: celestial vs terrestrial; objects have natural places determined by the four elements (Earth, Water, Air, Fire).
Natural motion: terrestrial objects move up or down; celestial bodies move in circular motion.
Violent (imposed) motion: requires external force to start and maintain motion, especially horizontal motion on Earth which is considered non-natural.
Cannonball example: continuous applied force would be needed to keep horizontal motion according to Aristotle.
Galileo challenged these ideas; argued that motion can persist without continuous force if friction/air resistance are negligible.

Galileo argued that an object in motion would continue moving in the absence of friction, and that gravity affects vertical motion rather than requiring a force to maintain horizontal motion.
This critique led to the concept of inertia: resistance to changes in motion, not a force itself.

Inertia is the property of matter that resists changes in motion; it is not a force.
Mass measures inertia; larger mass implies greater inertia; more resistance to changes in motion.
Mass is constant (the same mass everywhere).
Everyday example: heavier person or object harder to start standing or stop moving; in a moving car, occupants tend to keep moving when the car stops if not restrained by a seat belt.
Notion that inertia explains why an object keeps moving unless acted on by a net external force.

Statement: Every object remains at rest or in uniform straight-line motion unless acted upon by a nonzero net external force.
Mathematical sense:
$\mathbf{F}_{net} = 0 \Rightarrow \Delta \mathbf{v} = 0$
This law quantifies the inertia concept and introduces the idea of net force causing changes in motion.

Force: a push or pull; not the same as inertia.
Net force: vector sum of all forces acting on an object: $\mathbf{F}_{net} = \sum \mathbf{F}$
Acceleration arises from net force; not explicitly given here, but is the rate of change of velocity.
Velocity vs speed:
- Speed: scalar magnitude of velocity; $v = |\mathbf{v}|$
- Vector Quantity- Velocity: vector quantity with magnitude and direction.
Acceleration: rate of change of velocity; $\mathbf{a} = \frac{d\mathbf{v}}{dt}$

Support force (normal force): contact force perpendicular to a surface.
Friction: force opposing motion between surfaces; kinetic friction depends on normal force: $F<em>f = \mu</em>k N$ ; static friction: $F<em>f \le \mu</em>s N$

Aristotle vs Galileo: natural vs imposed motion; historical shift toward inertia.
Inertia: resistance to motion change; proportional to mass; not a force.
Mass is a measure of inertia; mass is constant across locations.
Newton's First Law: inertia plus net external force determines motion state.
Key definitions to memorize: speed, velocity, acceleration, net force, normal force, friction.