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Notes: Chapter 11.1 Newton’s 1 st & 2 nd Laws of Motion

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

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Notes: Chapter 11.1 Newton’s 1 st & 2 nd Laws of Motion

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  1. Notes: Chapter 11.1Newton’s 1st & 2nd Laws of Motion

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. Showing Motion • Vector: an arrow drawn toshow the direction of a force and the magnitude (size) of a force (in Newtons) • Diagrams like this are calledfree 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

  7. FN Free Body Diagrams Ff Fa Fg • Fais applied force (push/pull by you) • Fgarrow is gravity, always towards center of the Earth • Ffarrow is friction/air resistance. It is always opposite motion. • FNarrow is normal force; the surface supporting it. It is always perpendicular to surface. If something is “flying” there is no normal force

  8. Free Body Diagrams FN • Object to left is moving at constant speed or sitting still • Object to left is accelerating • Object to left is slowing down (negative acceleration) Ff Fa Fg FN Fa Ff Fg FN Ff Fa Fg

  9. 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 Ff Fg Ff FN Fg

  10. “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.

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