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Newton’s Second and Third Laws. Chapter 4 Section 3. Newton’s First Law. From Newton’s 1 st Law of Motion an object with balanced external forces acting on it is in a state of equilibrium. Σ F = 0 No acceleration

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newton s first law
Newton’s First Law
  • From Newton’s 1st Law of Motion an object with balanced external forces acting on it is in a state of equilibrium.
    • ΣF = 0
    • No acceleration
  • If the Forces are not balanced then there is a change in the motion of the object.
    • ΣF ≠ 0
    • Acceleration occurs
acceleration and force
Acceleration and Force
  • Acceleration is directly Proportional to the Force
    • Acceleration ~ Force
  • If the Force is increased, then the acceleration must increase by the same ratio as long as mass is held constant.
force and acceleration
Force and Acceleration
  • Acceleration is always in the direction of the net force.
acceleration and mass
Acceleration and Mass
  • Acceleration is inversely proportional to the mass of the object.
    • Acceleration ~ 1 / Mass
  • If the mass increases, then the acceleration decreases as long as the force remains constant.
    • If the mass is doubled, then the acceleration is cut in half.
force mass and acceleration
Force, Mass and Acceleration
  • The acceleration is directly proportional to the Force divided by the Mass
    • Acceleration ~ Force / Mass
  • This is where Newton’s 2nd Law is created from.
newton s 2 nd law of motion
Newton’s 2nd Law of Motion
  • Newton’s Second Law – The acceleration of an object is directly proportional to the net external force acting on the object and is inversely proportional to the mass of the object.
    • ΣF = ma
equation variables and units
Equation Variables and Units
  • Newton’s Second Law variables
    • Σ: Greek Letter Sigma meaning “The sum of”
    • F: Force (Newton – N)
    • m: Mass (Kilograms – kg)
    • a: Acceleration (meters per second² - m/s²)
what is a newton
What is a Newton?
  • A Newton is the amount of force needed to move a 1 kilogram mass at an acceleration of 1 meter per second squared.

F = ma

N = kg • m/s²


example problem
Example Problem
  • What force is needed to move a 3.2kg book across a table with an acceleration of 2.1 m/s² to the right?
  • Answer: 6.7 N to the right
solving problems with multiple forces
Solving Problems With Multiple Forces
  • It is often easier to break the Newton’s 2nd Law into components.
    • The sum of the forces in the x-direction equals the mass multiplied by the acceleration in the x-direction.
      • ΣFx = max
    • The sum of the forces in the y-direction equals the mass multiplied by the acceleration in the y-direction.
      • ΣFy = may
net external force equals zero
Net External Force equals Zero
  • If the net external force is zero, then the acceleration is equal to zero regardless of how much mass is present.
    • ΣF = ma
    • ΣF = m • 0m/s²
    • ΣF = 0
newton s 3 rd law
Newton’s 3rd Law
  • Newton’s Third Law – If two bodies interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.
  • For every action, there is an equal and opposite reaction.
forces always exist in pairs
Forces Always Exist in Pairs
  • Forces always exist in pairs, therefore there can not be a single isolated force.
    • If you push on a wall with 100N, the wall presses back on you with 100N.
      • Equal and opposite, as long as there is no acceleration.
  • If Earth is pulling you down with a force equal to your weight, what is the second force?
action reaction pair
Action-Reaction Pair
  • Action-Reaction Pair – A pair of simultaneous equal but opposite forces resulting from the interaction of two objects.
    • The action and reaction occur at the same exact time.
field forces
Field Forces
  • Field Forces also exist in pairs as well.
    • Field forces such as gravity and electromagnetism.
  • If you drop a ball the earth pulls down on the ball, but the ball pulls up on the earth by the same amount.
  • But why doesn’t the earth move and the ball does?
example problems 1
Example Problems #1
  • The net external force on the propeller of a 0.75kg model airplane is 17N forward. What is the acceleration of the airplane?
example problem 2
Example Problem #2
  • A ball pushed with a force of 13.5N accelerates at 6.5m/s² to the right. What is the mass of the ball?
example problem 3
Example Problem #3
  • Two people push on a box resting on a frictionless floor. One person pushes to the left with a force of 17N and the other person pushed with a force of 37N to the right. If the mass of the box is 10kg, what is the acceleration of the box?