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Transcript
Notes: Chapter 11.1 Newton’s 1st & 2nd Laws of Motion Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force • Things don’t change their motion (or rest) unless there are unbalanced forces • Also known as the Law of Inertia. Inertia • Inertia: tendency of object to not change motion. • It’s a property of matter, like density or color or flammability. Not really a number we measure, like forces. • More mass you have, more inertia you have e.g. takes more force to move something heavy because it has more inertia (resists that change) Newton’s Second Law of Motion • The unbalanced force acting on an object equals its mass times its acceleration. • F = ma • Really what it is saying is a =F/m • acceleration is directly proportional to force, inversely proportional to mass. Newton’s Second Law of Motion • Force measured in kg x m/s2 • Also called a Newton (N) • Mass and acceleration are opposites (if force stays the same). As one gets bigger, the other gets smaller. Showing Motion • Vector: an arrow drawn to show the direction of a force and the magnitude (size) of a force (in Newtons) • Diagrams like this are called free body diagrams: – vectors are drawn from the center of an object, even if that’s not where the force acts. • Remember: if the forces are balanced, there is no acceleration (still can have motion, just is constant velocity or at rest) • If forces are unbalanced, object will accelerate FN Ff Fa Free Body Diagrams Fg • Fa is applied force (push/pull by you) • Fg arrow is gravity, always towards center of the Earth • Ff arrow is friction/air resistance. It is always opposite motion. • FN arrow is normal force; the surface supporting it. It is always perpendicular to surface. If something is “flying” there is no normal force Free Body Diagrams FN Ff Fa • Object is moving at constant speed or sitting still Fg FN Fa Ff Fg FN Ff Fa Fg • Object is accelerating to the right • Object is slowing down while moving to the right (negative acceleration) Free Body Diagrams • What would something look like falling? Ff – no normal…no surface – no applied…no push Fg Ff FN Fg • What would something look like going down a ramp? – normal…perpendicular to surface – no applied…no push – gravity straight down – friction opposite motion “Ignore friction/air resistance” • A lot of times, we’ll talk about “ignoring friction” or ignoring “air resistance.” This is “ideal world” where things are perfect. It tells us what should happen. • When we do experiments, we may not always see the same results, because we are in the real world where there is friction and air resistance.
