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Today’s Topics • Newton’s Three Laws • Mass and Weight • Free Body Diagrams Newton’s 2nd Law A _______ is that which causes (unrestrained) objects to __________ Links forces to motion Newton’s 1st Law We frequently can have knowledge of forces acting on an object NET Force: ________________ If no NET _______ is applied to an object… the object either remains at _______, OR moves at a _________________ “At rest” is a special case of constant v, with v = 0! Since _______ remains constant unless a net force is applied, objects appear to be “_______” Latin word for laziness: ___________ 1st Law sometimes referred to as: Can use the 2nd Law to translate the force into acceleration Can then find velocity and position Fnet=ma Force: Scalar or vector? Mass: Scalar or vector? Rolling cart – constant velocity? EF 151 Spring, 2017 Lec 2-3 1 EF 151 Spring, 2017 Lec 2-3 SI Units of Acceleration, Mass & Force Newton’s 3rd Law If one object exerts a force on a second object, the ________ object exerts a force on the _______ object Basis This “_________” force has a magnitude equal to the first force g (at earth’s surface) This “_________” force acts in the _________direction as the first force Quantity SI System USC System Mass-based Force-based 9.81 m/s2 32.2 ft/s2 Mass kg slug Force N pound F=ma 1N = 1kg(1m/s2) 1lb = 1slug(1ft/s2) SI System: kg linked to meter through H2O Density of water is 1000 kg/m3 = 1 kg/L = 1 g/cm3 How are you able to stand on the floor? How are you able to jump up off the floor? USC System: two other force units 1 ton = 2000 lb 1 kip (kilopound) = 1000 lb A couple of weeks ago we dropped a 10 ton wrecking ball onto a Mini Cooper. Which object experienced the greater force during that collision? EF 151 Spring, 2017 Lec 2-3 2 3 EF 151 Spring, 2017 Lec 2-3 4 FBD = KD Conversion Factors / Weight Force 1 Newton = 0.2247 lb (~1/4 lb) 1 lb = 4.448 Newtons (~4.5 N) What is a 200 lb professor’s weight in Newtons? _______ ____________________ Diagram of ______ free from surroundings and showing all ________ that act on the body Mass 1 slug = 14.594 kilograms What is a 30 kg dog’s mass in slugs? _______ Weight: force of gravity W = ______ Mass of an object is same everywhere - weight depends on location 200 lb on earth has a mass of ________ 30 kg on earth has a weight of ________ Identify ___________ Draw ___________ Define a ______________ Draw all ________ that act ___ the body Do not show ________________ = = _____________________ Diagram of ______ free from surroundings and showing ___________________ (inertial) vectors Draw same body Show all __________ components Runner starting a race: Lazydog weighs 64 lb on the earth and gmoon is 5.31 ft/s2 what is the dog’s mass on the moon? __________ slugs __________ kg Ball being thrown: what is the dog’s weight on the moon? ___________ lb ___________ N EF 151 Spring, 2017 Lec 2-3 5 The Original Alabama Bass Tracker EF 151 Spring, 2017 Lec 2-3 6 The Original Alabama Bass Tracker Alabama fan weighs 195 lbs. His “boat” and motor weigh 218 lbs and the motor provides a force of 143 lbs. The drag resistance of the water is 62 lbs. Alabama fan weighs 195 lbs. His “boat” and motor weigh 218 lbs and the motor provides a force of 143 lbs. The drag resistance of the water is 62 lbs. What is the acceleration of the boat? What are the forces on the fisherman? What mass is used for kinetic diagram? EF 151 Spring, 2017 Lec 2-3 7 EF 151 Spring, 2017 Lec 2-3 8