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Transcript
Forces Today’s Goal: 1. Describe and give examples of the law of inertia and understand that inertia is a basic property of matter Sir Isaac Newton • English “natural philosopher” (what we would call a scientist. • Famous apple • Published in 1687 The Philosophiae naturalis principia mathematica, commonly known as the Principia • In the King’s English: The Mathematic Principles of Natural Philosophy The Principia • First publication of its kind; nobody had ever treated science so rigorously • In order to adequately describe nature, Newton invented Calculus at the same time • Laid out the fundamental principles of Classical Physics which dominated science until the advent of Quantum Mechanics in the 1920’s Newton • Also developed the science of optics • Was knighted (the first scientist to be so honored) • “If I have seen so far, it is because I stood on the shoulders of Giants.” Mass • The quantity of matter – how much stuff something is made of • On a fundamental level dependent on the number of atoms that make something up • SI Unit : kilograms • An object’s mass can only be changed by changing the object (breaking it, cutting, etc) Forces • A force is any action that is capable of accelerating an object – that is capable of changing the motion of an object • Push or pull • Forces are vectors; meaning they have magnitude and direction • Symbol: F • http://www.brainpop.com/science/motionsforce sandtime/force/ Newton’s Second Law • The acceleration of an object is proportional to the net force acting on the object and is inversely proportional to the mass of the object. • Net force – sum of all forces acting on an object. F a m F ma Newton’s First Law • So what if no net force acts on an object? – The object continues in motion in a straight line with constant velocity (the acceleration is zero) – If it’s at rest, it stays at rest. • Inertia: the tendency of any object to resist changes in its motion • Equilibrium – state when the net force acting on an object is zero Newton’s First Law • “Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.” Newton Newton’s First Law • The idea that an object keeps moving forever in a straight line is contrary to what you see. This is because every object that you encounter on a daily basis is stopped by friction or drag. Newton’s Third Law • Forces always occur in pairs • When you push on something, it pushes back. This force is called a reaction force. • The reaction force is equal in magnitude to the original force & opposite in direction. Measuring Force • Forces can be measured with springs or other elastic objects (such as rubber bands) • SI Unit: Newton (N) • English Unit: Pound (lbs) 1 lb = 4.4 N • Another unit: ton 1 ton = 2000 lbs Force Magnitudes Force (N) Force (lbs) Refrigerator magnet 10 2.3 Weight of a person 660 150 Force on jet pilot during takeoff Force generated during space shuttle launch 2600 600 140,000 35,000 Two Kinds of Forces • Long – range forces – act over distances between two objects • Contact forces – force between two objects in direct contact • What are some examples of each? Four Fundamental Forces 1. Gravity – acts over distances between two objects 2. Electromagnetic – responsible for all the forces you encounter except gravity – force between charged particles 3. Weak Nuclear Force 4. Strong Nuclear Force Hold nucleus of atoms together: very short range Forces You Should Know • Gravity (Fg) – the force of attraction between two objects: also known as weight • Tension (FT) – the force that holds a string or rope together • Friction (Ff) – force caused by surfaces rubbing together; acts to oppose relative motion • Normal (FN) – reaction force of a surface on anything pushing on it Forces You Should Know • Drag (FD) – force of a fluid exerted on anything moving through it (air resistance) • Thrust (FT) – force from engines, rockets, etc. that propel an object forward • Lift (FL) – Upward force that causes airplanes and other flying objects to fly What is Gravity? GRAVITY: An attraction force between all masses Newton’s universal law of gravitation: Every object in the universe exerts a gravitational attraction to all other objects in the universe The amount of gravitational force depends upon the mass of the objects and the distance between the objects Mass vs. Weight • Mass – the measure of how much matter an object possess • Weight – a measure of how strongly the Earth (or whatever planet an object is on) pulls the object down: same as saying “force of gravity” • Mass and Weight ARE NOT THE SAME!! Mass vs. Weight • Mass can only be changed by adding or removing matter from the object • Weight can be changed by moving to a different planet, or by changing the mass • Weight is dependant on the mass of an object and the acceleration due to gravity, which changes from place to place. Fg mg