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Transcript
Hey! Your clicker wants to know... Quick Quiz Did you read the chapter?! Which of the following is going to be influenced by Strong interactions? A. B. C. D. E. galaxy quark solar system person molecule (A) Yes (B) No 1 Main Ideas natural motion: how things move in the absence of external forces Newton’s Second Law of Motion Force Newton’s First Law of Motion 2 how an object moves in response to an applied force Newton’s Third Law of Motion a push or pull on one object by another anything that causes a change in motion measured in Newtons (pounds) Has a specific direction Has nothing to do with motion Tells where forces come from 3 Net Force Identifying forces The sum of all the forces acting on an object. Fundamental forces – Also called the “resultant force” or total force. Forces must be added as vectors 4 2 north + 2 south = 0 2 north + 2 east = √8 north east 5 PS100, Sect. 3, Winter 2000 Does the object in question have mass? Does the object in question have charge? Contact forces – can act at a distance. anything touching our object Normal forces Friction Tension 6 Chapter 3, Page 1 Newton’s First Law: When does 1st law apply? Unless acted on by a net external force, an object will continue in motion at a uniform speed and in a uniform direction If the total force is zero. No net force Æ No change in motion. If there are no forces If the forces balance If the object is at rest. If the object is moving in a straight line at constant speed. (Uniform Motion) 7 8 Acceleration Newton’s Second Law Force = mass x acceleration velocity—both speed and direction of motion acceleration--change in a object’s velocity Acceleration is any change in speed or direction. 9 10 Mass a property of objects that determines how they respond to a force different from weight Newton’s Second Law weight is a force that depends on gravity objects in space still have mass How much an object’s motion changes depends on how hard you push on it, and how big the object is. stronger forces cause more acceleration larger mass results in less acceleration acceleration is in the direction of the net force measured in grams or kilograms (slugs) 11 PS100, Sect. 3, Winter 2000 12 Chapter 3, Page 2 When does Sally Discovers Newton’s Third Law law apply Any time there is a net force. Any time the motion of an object is changing 2nd Change in speed Change in direction Note an object is always either in accelerated or in uniform motion, so either the 2nd law or 1st law applies. 13 14 Newton’s Third Law When do you use 3rd law forces come from two interacting objects (objects cannot act on themselves) there are forces on both objects these forces are equal in magnitude and opposite in direction Rules: 1. Just because two forces are equal and opposite does NOT mean they are an action-reaction pair. ¾ An action-reaction pair acts on a single pair of objects whose roles switch. 2. The 3rd law of motion doesn’t have anything to do with motion. ¾ A 3rd law pair measures the strength of a single interaction, NOT net force. 15 16 Newton’s Third Law Examples firing a shotgun woman standing on the floor bouncing a basketball playing a guitar rock climbing (“pull yourself up?”) drag racing 1. 2. 3. 17 PS100, Sect. 3, Winter 2000 Demo Questions Identify the forces Decide whether the 1st or 2nd law applies Decide if the 3rd law is pertinent 18 Chapter 3, Page 3 Summary Cartoon Laws of Physics Think of some examples of how Newton’s Laws of Motion would be different if written by cartoon animators (Road Runner cartoons, for instance). 19 PS100, Sect. 3, Winter 2000 Newton’s Laws Force Mass Acceleration 20 Chapter 3, Page 4