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Newton’s 3rd law of motion
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Newton’s 3rd law of motion



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  • 1. Newton’s 3 rd Law of Motion
  • 2. A. Forces
    • Forces never act alone, but instead as pairs
    • Interaction (inter – between) An action between objects where each object exerts an equal & opposite force on the other.
  • 3.  
  • 4.  
  • 5.  
  • 6. B. Newton’s 3 rd Law
    • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
    • For every action there is an equal & opposite reaction.
  • 7. C. Action/Reaction of Different Masses
  • 8. Firing a cannon!!! BOOM!!!
    • When the cannon is fired, there is an interaction between the cannon and the cannonball.
    • Their forces are equal and opposite. (look at the size & direction of the arrows in the diagram)
    • The cannon has a “kick” in the opposite direction of the cannonball.
    • Why doesn’t it move further than just a “kick”?
  • 9. Compare the masses of the objects
  • 10. IV. Free Fall
    • Galileo studied falling objects
    • He showed that falling objects accelerate equally regardless of their mass.
    • He could never say why the accelerations were equal.
  • 11. Apply this to Newton’s 2 nd Law
    • Remember, mass & weight are proportional. i.e. a 2kg cat weighs twice as much as a 1kg cat
    • Using Newton’s 2 nd law we consider mass
    • 2x as much force acting on 2x as much mass will produce the same acceleration as a smaller force acting on a smaller mass
    • The ration of force to mass stays the same no matter the size of the object.
  • 12.  
  • 13. V. Falling & Air Resistance
    • Coin vs. feather
    • In a vacuum they fall with equal acceleration
    • In the presence of air, they react differently to air resistance
  • 14.
    • Air resistance diminishes the net forces acting on the object
    • The net forces decrease very little for the coin, but a lot for the feather.
    • The air resistance builds up quickly and cancels the weight force of the feather.
  • 15.
    • When the air resistance = weight, the net force is zero.
    • Fnet = 0, there is no acceleration
    • Terminal velocity – the speed at which the acceleration of a falling object is zero because friction balances the weight.
    • A heavier object will attain a greater terminal velocity than a lighter object.
    • It appears to fall faster.