Newton's Second Law

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  • Created by: Agata
  • Created on: 24-02-16 18:11
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  • Newton's Second Law
    • Newton's Second Law: The rate of change of the momentum  of an object is directly proportional to the net force acting upon it.
      • ?p/?t ? F
        • mv-mu/ t ? F   this gives: m(v-u)/t ? F, hence ma ? F
          • Note the proportionality sign. In order to make this an equation, a constant of proportionality k must be introduced.
            • This gives Newton's Second Law as ma=kF
              • Newton's Second Law: The rate of change of the momentum  of an object is directly proportional to the net force acting upon it.
                • ?p/?t ? F
                  • mv-mu/ t ? F   this gives: m(v-u)/t ? F, hence ma ? F
                    • Note the proportionality sign. In order to make this an equation, a constant of proportionality k must be introduced.
                      • This gives Newton's Second Law as ma=kF
      • One Newton - The force that will give a mass of one kilogram an acceleration of one meter per second each second.
        • 1 kg x 1m/s^2 = k x 1N
      • Mass - The inertia of an object, or how difficult it is to accelerate the object.             Units: kilograms
      • Weight - The gravitational force the Earth exerts on an object, pulling it towards the ground.           Units= N          On Earth W=M x 9.81 m/s^2.
      • WORKED EXAMPLE: In an experiment to measure the charge on an electron, an electrical force of 2.8 x 10 ^-8 N is exerted upwards on a charged oil drop of mass 3.0 micro grams. What is the acceleration of the oil drop?
        • 1) The mass needs to be in kg.    3.0 micro grams= 3.0 x 10 ^-6 g = 3.0 x 10 ^-9 kg.
          • 2) The weight W of the drop is (3.0 x 10^-9)  x 9.81= 2.943 x 10^-8 N
            • 3) This gives  a resultant downward force on the drop as  (2.943 x 10^-8 ) - (2.8 x 10^ -8) = 0.143 x 10 ^-8 N.
              • 4) F(down) = m x a(down)      to get                 (0.143x 10^-8) = (3.0 x 10^-9) x a
                • 5) a (down) = (0.143 x 10^-8)/ (3.0 x 10^-9) = 0.477 m/s^2

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