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Transcript
Unit 3 - Forces Newton’s Laws of Motion Different Forces Applying Newton’s Laws Work and Energy Momentum and Collisions Using Mass & Weight • With study groups, measure weight of 3 different masses ( using force scale and mass hangers ) – 100, 200, & 300 g • Plot weight measured ( in Newtons ) vs mass • Objective: what is the proportionality constant? • Use to discuss Newton’s Laws Free Body Diagrams • First, draw a sketch in your notes of a mass hanging from a force scale • What are the forces acting on the mass? • What are the forces acting at your shoulder? Measure and Plot ( Notes ) • Get 3 100 g masses – will measure 100, 200, 300 g with force scale ( make a table with “g” and “N” ) – MAKE SURE FORCE SCALE IS INITIALLY READING ZERO • Sketch a graph in your notes, plotting weight measured vs. mass hung on force scale • What is the slope? • Formulate an equation for the line plotted Force and Mass Force: push or pull Force is a vector – it has magnitude and direction Force and Mass Mass is the measure of how hard it is to change an object’s velocity ( INERTIA ). Mass can also be thought of as a measure of the quantity of matter in an object. Newton’s First Law of Motion If you stop pushing an object, does it stop moving? Newton’s First Law - In the absence of any net external force, an object will keep moving at a constant speed in a straight line, or remain at rest. This is also known as the law of inertia. In order to change the velocity of an object – magnitude or direction – a net force is required. Hanging Mass • Is there a NON-ZERO net force acting on the hanging mass? • So, is the mass in motion? Newton’s Second Law of Motion Two equal weights exert twice the force of one; this can be used for calibration of a spring: Newton’s Second Law of Motion What if we drag an object with the force scale? Acceleration is proportional to force: Newton’s Second Law of Motion Acceleration is inversely proportional to mass: Newton’s Second Law of Motion Combining these two observations gives Or, more familiarly, Newton’s Second Law of Motion An object may have several forces acting on it; the acceleration is due to the net force: (5-1) Newton’s Second Law of Motion Free-body diagrams: A free-body diagram shows every force acting on an object. • Sketch the forces • Isolate the object of interest • Choose a convenient coordinate system • Resolve the forces into components • Apply Newton’s second law to each coordinate direction Newton’s Second Law of Motion Example of a free-body diagram: Newton’s Third Law of Motion Forces always come in pairs, acting on different objects: r r If object 1 exerts a forceF on object 2, then object 2 exerts a force –F on object 1. These forces are called action-reaction pairs. What was the action-reaction pair on the hanging mass? What about on your shoulder as you hold the force scale? Newton’s Third Law of Motion Every action has an equal and opposite reaction Newton’s Third Law of Motion Some action-reaction pairs: