Newton's Second Law

• Created by: Agata
• Created on: 24-02-16 18:11
• 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