Chapter 4 Newton’s Laws and Motion I.History of Explaining Motion A.Aristotle’s Motion (384-322 B.C.) 1)Aristotle was a Greek “philosopher” 2)Science by.

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  • Chapter 4 Newtons Laws and MotionHistory of Explaining Motion

    Aristotles Motion (384-322 B.C.)Aristotle was a Greek philosopher

    Science by observation, then thinking, not experimentationHis ideas might match some of your initial conceptions of physics

    Force = push or pull on an objectMust have a force to have motion

    Velocity is directly proportional to strength of forceHeavy object falls faster than light objectStrength of the force = heaviness of the object

    Velocity is inversely proportional to the resistance of the medium (air water)Aristotles IdeasNot allCorrect

  • 4) Direct and Inverse proportions: how two variables are relatedDirectly Proportional: If A increases, B increases

    Inversely Proportional: If A increases, B decreases

    Velocity and Acceleration were the same thing to Aristotle

    How does a ball keep going once it leaves your hand?Motion requires a constant forceAir filling a vacuum behind the ball pushes it forwardReasonable answer, but not experimentally true

    If velocity increases, distance increases: if t = 2 s find d for v1 = 10 m/s and v2 = 11 m/sIf time increases, velocity decreases: if d = 30 s find v for t1 = 2 m/s and t2 = 11 m/s

  • Galileos Motion (1564-1642)Aristotles explanations were thought of as absolute truthGalileo performed experiments testing these explanationsg is the same for all objectsObservations favored Sun-centered Solar SystemProposed an object in motion stays in motion without a constant forceMathematical Description of MotionAcceleration was different from Velocity

    Newtons Motion (1642-1727)Theory of motion that matched experimentsWorked for all objects: planets or dropped ballsNo difference between heavenly and earthly objectsLed to predictions not yet observed (Planet Neptune)Three Laws of Motion (rest of the chapter)Universal GravitationInvented CalculusD. Science builds on the work of those before you

  • Newtons First and Second LawsNewtons First Law of MotionAn objects velocity changes only if acted on by a forceAn object at rest stays at rest unless acted on by a forceAn object in motion maintains its velocity unless acted on by a force

    Aristotle: thrown ball or ice skater must have constant forceNewton: only needs a force to stop once it is going

    3)What eventually stops a moving object?Friction: force opposing motion due to physical interaction of surfacesExamples: Air Resistance, Road/Tire interaction

  • B. Newtons Second Law of MotionAcceleration is directly proportional to force and inversely proportional to mass.

    Acceleration, not velocity, is the property affected by a forceForce = interaction of one object with another causing accelerationInertia = property of objects to resist changes in motionMass = measure of inertia; how much object resists change in motionUnits = Kilogram = kgUnits of Force are called Newtons (N)

  • Adding ForcesForce is a vector quantity: magnitude and directionOften multiple forces act on an object at the same timeTotal Force = Net Force = sum of the individual forcesFriction is a common force we must deal with in finding Net Force

    m = 5 kga ?

    D. The 1st law is a special case of the 2nd law when Force = 0

  • Mass and WeightComparing MassesUse 2nd Law to define mass

    Compare masses by comparingAcceleration caused by equal forces

    WeightUse g for comparison of massesWeight = gravitational force acting on an object

    m = 50 kg. What is the weight?1 lb. = 4.45 N

    How many lbs in a kg?

  • Mass and WeightMass doesnt change; it only depends on F and aWeight depends on g, which changes depending on where you areHow much does a 110 lb person weigh on the moon? (gm = 1.6 m/s2)

    Why is gE independent of mass?

    Force and acceleration are not the sameHeavy object has larger weight(force) than a light objectHeavy and light objects accelerate at g


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