Newtons 2nd and 3rd Law

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					Newton’s   2 nd   and   3rd   Law
  A. Newton’s 2nd Law
  B. Newton’s 3rd Law
       C. Project
       A. Newton’s 2nd Law

 The acceleration of an object is dependent upon
  two variables:
  – Fnet acting on the object
  – Mass of the object
 The acceleration of an object depends directly
  upon the net force acting upon the object and
  inversely upon the mass of the object
 As Fnet increases, acceleration increased
 As mass increases, acceleration decreases
A. Newton’s 2nd Law
        A. Newton’s 2nd Law

 The net force is equal to the product of the
  object's mass and its acceleration.
  – Fnet = ma


 Acceleration is in the same direction as Fnet
 Remember: the net force is the vector sum
  of all the forces
       A. Newton’s 2nd Law

 What acceleration will result when a 12-N
  net force is applied to a 3-kg object? A 6-kg
  object?

 A net force of 16 N causes a mass to
  accelerate at the rate of 5 m/s2. Determine
  the mass.
       A. Newton’s 2nd Law

Net Force
   • Fnet = ma
Gravitational Force
   • Fgrav = mg
Frictional Force
   • Ffrict = μ(Fnorm)
     A. Newton’s 2nd Law
 An applied force of 50 N is used to accelerate an
  object to the right across a frictional surface. The
  object encounters 10 N of friction. Use the diagram
  to determine the normal force, the net force, the
  mass, and the acceleration of the object. (Neglect air
  resistance.)
      A. Newton’s 2nd Law
 An applied force of 50 N is used to accelerate an object to
  the right across a frictional surface. The object encounters 10
  N of friction. Use the diagram to determine the normal force,
  the net force, the mass, and the acceleration of the object.
  (Neglect air resistance.)


Fnorm = 80 N
m = 8 kg
a = 5 m/s2
Fnet = 40 N,
       right
      A. Newton’s 2nd Law
 An applied force of 20 N is used to accelerate an object
  to the right across a frictional surface. The object
  encounters 10 N of friction. Use the diagram to
  determine the normal force, the net force, the
  coefficient of friction (µ) between the object and the
  surface, the mass, and the acceleration of the object.
  (Neglect air resistance.)
      A. Newton’s 2nd Law
 An applied force of 20 N is used to accelerate an object
  to the right across a frictional surface. The object
  encounters 10 N of friction. Use the diagram to
  determine the normal force, the net force, the
  coefficient of friction (µ) between the object and the
  surface, the mass, and the acceleration of the object.
  (Neglect air resistance.)

      Fnorm = 100N
      μ = 0.1
      m = 10 kg
      a = 1 m/s2 right
      Fnet = 10N right
      A. Newton’s 2nd Law
 A 5-kg object is sliding to the right and encountering a
  friction force which slows it down. The coefficient of
  friction (µ) between the object and the surface is 0.1.
  Determine the force of gravity, the normal force, the force
  of friction, the net force, and the acceleration. (Neglect air
  resistance.)
      A. Newton’s 2nd Law
 A 5-kg object is sliding to the right and encountering a
  friction force which slows it down. The coefficient of
  friction (µ) between the object and the surface is 0.1.
  Determine the force of gravity, the normal force, the force
  of friction, the net force, and the acceleration. (Neglect air
  resistance.)

     Fnorm = 50N
     Ffrict = 5N
     Fgrav = 50N
     m = 5kg
     a = 1 m/s2
     Fnet = 5N left
      B. Newton’s 3rd Law

Whenever one object exerts a force on
 a second object, the second object
 exerts an equal and opposite force on
 the first.

Action-reaction pairs never act on
 same object.
       B. Newton’s 3rd Law

 Two objects define a system for a Newton’s
  third law interaction.
 We are not considering (necessarily) the net
  force acting on an object.
 An object cannot exert a force on itself to
  cause an acceleration.
    Newton’s 3rd Law
Action and Reaction on Different Masses

      Consider you and the earth


                       Action: earth pulls on you




                       Reaction: you pull on earth
         B. Newton’s 3rd Law




Reaction: road pushes on tire   Action: tire pushes on road
              B. Newton’s 3rd Law




                          Reaction: gases push on rocket


Action: rocket pushes on gases
   B. Newton’s 3rd Law
A bug and a car collide. Which experiences
the greater force?
(a) bug
(b) car
(c) neither, they both experience the same
    magnitude of force
   B. Newton’s 3rd Law
Consider hitting a baseball with a bat. If we
call the force applied to the ball by the bat the
action force, identify the reaction force.

(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
       C. Project: Balloon Rocket
                   Racer
 Design a small car powered by a balloon
  and explain how the third law of motion
  describes its propulsion

      DUE:

				
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posted:7/2/2013
language:English
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