Physics for Engineers 1: Notes on Newton's Laws of Motion

Physics for Engineers 1 Lecture: Newton's Laws of Motion

Objectives

  • Describe the effects forces have on objects.
  • Draw free-body diagrams to show relationships of forces acting on objects.

Outline

  1. Types of Forces
  2. Free-Body Diagrams

Types of Forces

  • Gravity Force:

    • The force of gravity acting downward.
    • Denoted by W.

  • Friction Force:

    • Opposes motion, dependent on normal force.
    • Denoted by Ff or f.

  • Applied Force:

    • A push or pull exerted on an object.
    • Types:
    • Push Force: Separates objects.
    • Pull Force: Moves objects together.

  • Normal Force:

    • Force exerted by a surface to support the weight of an object resting on it.
    • Denoted by FN or N; always perpendicular to the surface.
    • Prevents objects from falling through surfaces.

  • Tension Force:

    • Force exerted by a rope or cable.
    • Denoted by FT or T.

  • Buoyant Force:

    • The upward force exerted by a fluid on an object submerged in it.

  • Magnetic Force:

    • The attraction or repulsion between magnetic materials.

Free-Body Diagram (FBD)

  • A graphic representation showing all the forces acting on an object.
  • Focuses on a single object to visualize forces like pull, friction, and gravity.
  • Must only include forces acting on the object, excluding forces produced by it.

Conditions of Equilibrium

  • Equilibrium: Classification when object is at rest or moving with constant velocity.
  • Characteristics of Equilibrium:
    • Total (net) force acting on an object is zero: ( \sum Fx = 0 ) and ( \sum Fy = 0 )
    • Total moments acting on the object is zero: ( \sum M = 0 )

Balanced and Unbalanced Forces

  • Balanced Forces: Do not cause motion change; resulting force = 0.
  • Unbalanced Forces: Cause changes in motion; resulting force ≠ 0.

Newton’s Laws of Motion

First Law (Law of Inertia)
  • An object at rest will remain at rest, and an object in motion will maintain its uniform motion unless acted upon by a net external force.
  • Example: An unbelted passenger in a car continues moving forward during a sudden stop.
Second Law (F = ma)
  • Acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
  • Formula: ( a = \frac{F_{net}}{m} )
  • Example Calculation: If an object has a mass of 1800 kg and an acceleration of 3.8 m/s², the force acting on it is calculated as: ( F = ma = 1800 \times 3.8 = 6840N ).
Third Law (Action-Reaction Law)
  • For every action, there is an equal and opposite reaction.
  • Interaction of forces:
    • The force exerted by object A on object B is equal to the force exerted by object B on object A, but in the opposite direction.

Examples of Everyday Applications of Newton's Laws

  • A swimmer pushes on water; water pushes back.
  • A child pushing against the ground to jump; ground pushes back.

Practice Problems

  1. Force to cause deceleration on a rocket sled: ( F_{net} = 4.12 \times 10^5 N ) for mass = 2100 kg.
  2. Calculate acceleration of object when force applied remains constant, but mass is halved.
  • Understanding concepts: Examples of inertia and everyday situations involving forces serve to solidify understanding of these principles.

Key Connections

  • The interplay of balanced and unbalanced forces influence object motion, epitomized in Newton’s laws, creating a foundational understanding of motion principles necessary for engineering applications in physics.