Newton's Laws of Motion and Applications

Lesson Objectives

  • At the end of the discussion, students are expected to:
    • Compare and contrast theory and law.
    • Explain Newton’s Three Laws of Motion.
    • Give examples of the application of each law.

Newton’s Laws of Motion

  1. 1st Law of Motion (Law of Inertia)

    • Definition: An object at rest remains at rest and an object in motion remains in motion unless acted upon by an external unbalanced force.
    • Key Concept: Inertia is the tendency of an object to resist changes in its state of motion.

  2. 2nd Law of Motion (Law of Acceleration)

    • Definition: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
    • Formula: F = m imes a
      • Where,
      • F = Force (in Newtons, N)
      • m = Mass (in kilograms, kg)
      • a = Acceleration (in meters per second squared, m/s²)
    • Key Relationships:
      • Acceleration is directly proportional to force: a ext{ is directly proportional to } F
      • Acceleration is inversely proportional to mass: a ext{ is inversely proportional to } m

  3. 3rd Law of Motion (Law of Interaction)

    • Definition: For every action, there is an equal and opposite reaction.
    • Key Concept: Forces always occur in pairs; if object A exerts a force on object B, object B exerts an equal and opposite force on object A.

Theory vs Law

  • Theory:

    • Explains why natural phenomena occur.
    • Based on hypotheses, can be revised and used to make predictions.

  • Law:

    • Summarizes a set of observations about natural phenomena.
    • Represents a fundamental principle of nature that holds true under specific conditions.

Law of Inertia Applications

  • Examples of applications include:
    • A ball rolling on the ground will eventually stop due to friction.
    • A book on a table remains at rest until someone pushes it.

Law of Acceleration Applications

  • Example Calculations:
    • Example 1: A man pushes a 10-kg box forward at an acceleration of 2 ext{ m/s}^2.
    • Given: m = 10 kg, a = 2 m/s²
    • Formula: F = m imes a
    • Solution: F = 10 ext{ kg} imes 2 ext{ m/s}^2 = 20 ext{ N}
    • Answer: The man exerts a force of 20 N.
    • Example 2: Louisa rolls a 20-kg ball with a force of 31 N. To find its acceleration:
    • Given: m = 20 kg, F = 31 N
    • Formula: a = rac{F}{m}
    • Solution: a = rac{31 ext{ N}}{20 ext{ kg}} = 1.55 ext{ m/s}^2
    • Answer: The ball will accelerate at approximately 1.55 m/s².

Law of Interaction Applications

  • Examples of applications include:
    • A rocket taking off demonstrates action-reaction pairs (exhaust gases pushing down and rocket moving up).
    • Walking: Your foot exerts a backward force against the ground, and the ground exerts an equal and opposite force to push you forward.

Mass vs. Weight

  • Mass:
    • The amount of matter in an object, measured in kilograms (kg).
  • Weight:
    • The gravitational pull on that mass, measured in Newtons (N).
    • Formula: ext{Weight} = ext{mass} imes g
      • Where g is the acceleration due to gravity (approx. 9.81 ext{ m/s}^2 on Earth).

Challenge Question

  • If your mass on Earth is 85 kg:
    1. What is your weight on Earth?
    • ext{Weight} = 85 ext{ kg} imes 9.81 ext{ m/s}^2 = 833.85 ext{ N}
    1. What is your mass on another planet having gravitational pull one-fifth that of Earth?
    • Your mass would still be 85 kg since mass does not change with location, only weight does!