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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 We frequently can have knowledge of forces acting on an object Can use the 2nd Law to translate the force into acceleration Can then find velocity and position Newton’s 1st Law 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: Force: Scalar or vector? Mass: Scalar or vector? Fnet=ma Rolling cart – constant velocity? EF 151 Spring, 2016 Lec 2-3 1 EF 151 Spring, 2016 Lec 2-3 2 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 Quantity Basis g (at earth’s surface) This “_________” force has a magnitude equal to the first force Mass This “_________” force acts in the _________direction as the first force SI System USC System Mass-based Force-based 9.81 m/s2 32.2 ft/s2 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, 2016 Lec 2-3 3 EF 151 Spring, 2016 Lec 2-3 4 Conversion Factors / Weight Types of Forces Force Field Forces (act at a distance) 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? _______ _____________________ _____________________ Mass Contact Forces 1 slug = 14.594 kilograms What is a 30 kg dog’s mass in slugs? _______ Ropes, cables, string assumed to be flexible and of negligible mass; Weight: force of gravity tension the same everywhere 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 ________ force in the direction of the rope Normal force: ___________ Friction force: ______________ Lazydog weighs 64 lb on the earth and gmoon is 5.31 ft/s2 Spring force: ____________ what is the dog’s mass on the moon? __________ slugs __________ kg what is the dog’s weight on the moon? ___________ lb ___________ N EF 151 Spring, 2016 Lec 2-3 Fnet=ma 5 FBD = KD ____________________ Diagram of ______ free from surroundings and showing all ________ that act on the body Identify ___________ Draw ___________ Define a ______________ Draw all ________ that act ___ the body Do not show ________________ = EF 151 Spring, 2016 Lec 2-3 6 The Original Alabama Bass Tracker _____________________ Diagram of ______ free from surroundings and showing ___________________ (inertial) vectors 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? Draw same body Show all __________ components = Runner starting a race: Ball being thrown: EF 151 Spring, 2016 Lec 2-3 7 EF 151 Spring, 2016 Lec 2-3 8