Inertia, Forces, and Acceleration: The Legacy of Sir Isaac NewtonPosition is a “Vector”Speed vs. VelocityDiscussion QuestionsApproaching a Physics Question or ProblemNewton SaysSliding Book DemonstrationPowerPoint PresentationA Data TableAccelerationAcceleration is the rate of change of velocityAcceleration is a Vector tooDiscussion Questions, cont.Forces Cause AccelerationA Ball in Free FallQuestions on Newton’s Second LawQuantitative exercises, real numbersAnother numerical exampleSummaryAssignmentsUCSDPhysics 10Inertia, Forces, and Acceleration:Inertia, Forces, and Acceleration:The Legacy of Sir Isaac NewtonThe Legacy of Sir Isaac Newton Objects in MotionSpring 2008 2UCSDPhysics 10Position is a “Vector”Position is a “Vector”•CompareCompare ““A ball is 12 meters North of the Sun God”A ball is 12 meters North of the Sun God” to to ““A ball is 10 meters from here”A ball is 10 meters from here”•A vector has both a direction and a value, or “magnitude”A vector has both a direction and a value, or “magnitude”•Which of these descriptive properties are vectors?Which of these descriptive properties are vectors?–Position–Mass–Color–Speed–Velocity–TemperatureUsed interchangeably in casual language, but not in physicsSpring 2008 3UCSDPhysics 10Speed vs. VelocitySpeed vs. VelocitySpeed is the rate of motion (how fast)Speed is the rate of motion (how fast) Speed = distance / timeSpeed = distance / time “ “The satellite has a speed of 15,000 mi/hr”The satellite has a speed of 15,000 mi/hr” “ “But officer, my speed was only 56 miles per hour!”But officer, my speed was only 56 miles per hour!”Velocity is speed Velocity is speed plusplus directional information: directional information: “ “The spacecraft is moving at 18 km/sec The spacecraft is moving at 18 km/sec towards Jupitertowards Jupiter””Spring 2008 4UCSDPhysics 10Discussion QuestionsDiscussion QuestionsSee if you can come to a consensus on answers to See if you can come to a consensus on answers to these questions:these questions:1.1.A yellow car is heading East at 100 km/h and a A yellow car is heading East at 100 km/h and a red car is going North at 100 km/h. Do they red car is going North at 100 km/h. Do they have the same speed? Do they have the same have the same speed? Do they have the same velocity?velocity?2.2.A 16-lb bowling ball in a bowling alley in Del A 16-lb bowling ball in a bowling alley in Del Mar heads due north at 10 m/s. At the same time, Mar heads due north at 10 m/s. At the same time, a purple 8-lb ball heads due north at 10 m/s in an a purple 8-lb ball heads due north at 10 m/s in an alley in La Jolla. Do they have the same alley in La Jolla. Do they have the same velocity?velocity?Spring 2008 5UCSDPhysics 10Approaching a Physics Question or ProblemApproaching a Physics Question or Problem1.1.Extract relevant factsExtract relevant facts2.2.Draw a sketch, if neededDraw a sketch, if needed3.3.Determine applicable reasoningDetermine applicable reasoning4.4.Draw irrefutable conclusionDraw irrefutable conclusion5.5.Perform a “sanity check”. Does your answer Perform a “sanity check”. Does your answer make sense?make sense?Spring 2008 6UCSDPhysics 10Newton SaysNewton Says•A ball sitting still will stay that way, unless acted A ball sitting still will stay that way, unless acted upon by a force.upon by a force. An object that is not subjected to any outside forces An object that is not subjected to any outside forces moves at constant velocity, covering equal moves at constant velocity, covering equal distances in equal times, along a straight path, distances in equal times, along a straight path, xx((tt) = ) = xx(0) + (0) + vtvt Newton’s 1st LawInertia Mass• This is not intuitively obvious.Spring 2008 7UCSDPhysics 10Sliding Book DemonstrationSliding Book DemonstrationWhy doesn’t it keep on going, like the Energizer Bunny?When are there forces acting on the book, and what is responsible for them?When is the speed a maximum?When is the speed a minimum?How much force is acting on it after it stops?Spring 2008 8UCSDPhysics 10Constant Velocity Motion – No Forces•If no external forces are acting, velocity is constant•Position changes, at a steady (constant) ratex =1 m 2 m 3 m 4 m 5 m 6 m 7 mv= 1 m/s 1 m/s 1 m/s 1 m/s 1 m/s 1 m/s to rightHow does determination of velocity depend on choice x=0 and t=0?t=0 sec 1 sec 2 sec 3 sec 4 sec 5 sec 6 secSpring 2008 9UCSDPhysics 10A Data TableA Data TableTimeTime0 sec0 secPositionPosition1 meter1 meterVelocityVelocity= dist/time & direction= dist/time & direction1 sec1 sec2 meters2 meters1 m/s to right1 m/s to right2 sec2 sec3 meters3 meters1 m/s to right1 m/s to right3 sec3 sec4 meters4 meters1 m/s to right1 m/s to rightconstant velocitySpring 2008 10UCSDPhysics 10AccelerationAcceleration•If an object’s If an object’s velocityvelocity changeschanges, it’s accelerating. , it’s accelerating. •The change can be in the The change can be in the speedspeed of motion, in the of motion, in the directiondirection of motion, or both. of motion, or both.•Acceleration is a generic term Acceleration is a generic term  velocity change velocity change–includes “deceleration”•Can you Can you feelfeel if you’re moving with a constant if you’re moving with a constant velocity? velocity? •Can you Can you feelfeel being accelerated? Why? being accelerated? Why?Spring 2008 11UCSDPhysics 10Acceleration is the rate of change of velocityAcceleration is the rate of change of velocity•A A constant accelerationconstant acceleration means that the object’s means that the object’s velocity is changing at a constant ratevelocity is changing at a constant rate–Example: if the acceleration is along the direction of motion, the speed grows by the same amount in each time interval (e.g., second)•if the speed changes by 1 meter per second each second, the acceleration is (1 meter per second) per second, or 1 m/s2. if v = 15 m/s at time t = 0, and a = 1 m/s2, thenv = 16 m/s at t = 1 secv = 17 m/s at t = 2 secv = 20 m/s at t = 5 secSpring 2008 12UCSDPhysics 10Acceleration is a Vector tooAcceleration is a Vector too•Direction of acceleration = direction in which velocity Direction of acceleration = direction in which velocity