Notes: Chapter 11.1 Newton’s 1 st & 2 nd 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) HRW movie: mass and inertia

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/s 2 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

Free Body Diagrams F a is applied force (push/pull by you) F g arrow is gravity, always towards center of the Earth F f arrow is friction/air resistance. It is always opposite motion. F N arrow is normal force; the surface supporting it. It is always perpendicular to surface. If something is “flying” there is no normal force FaFa FNFN FgFg FfFf

Free Body Diagrams Object to left is moving at constant speed or sitting still Object to left is accelerating Object to left is slowing down (negative acceleration) FaFa FNFN FgFg FfFf FaFa FNFN FgFg FfFf FaFa FNFN FgFg FfFf

Free Body Diagrams What would something look like falling? – no normal…no surface – no applied…no push What would something look like going down a ramp? – normal…perpendicular to surface – no applied…no push – gravity straight down – friction opposite motion FgFg FfFf FfFf FNFN FgFg

“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.