Core Physics: Quantities, Motion, and Forces

Posted on May 9, 2025 in Physics

Physical Quantities

Physical quantities are properties of matter that can be measured.

Scalar Quantities

These are defined only by a number and the corresponding unit. Examples include temperature (T), mass (m), length, and pressure.

Vector Quantities

To define these, besides the number and the unit, it is necessary to add the direction and sense they possess. Examples include velocity, acceleration, force, and momentum.

Elements of a Vector

  • Module: The numerical value of the magnitude.
  • Direction: The line or straight path to which the vector belongs.
  • Sense: Indicated by the arrowhead of the vector.
  • Point of Application: The origin of the vector.

Fundamental Quantities

These are chosen arbitrarily as fundamental and are not defined in terms of any other quantities.

Understanding Motion

Reference Systems

These are defined by a coordinate system that determines the positions of a moving object (mobile), and a clock that measures time.

Inertial Reference Systems

An inertial reference system is one that, relative to another, is at rest or moves with uniform rectilinear motion. If the motion is not uniform and rectilinear, the system will not be inertial.

Characteristics of Movement

  • Position: The vector connecting the origin (0) of the reference system with the moving object (mobile) at any moment.
  • Trajectory: The path traced by the moving object.
  • Displacement: The vector joining the positions of the moving object at two different times.
  • Distance Traveled (Space Travel): The length measured along the trajectory. The distance traveled (s) only matches the magnitude of the displacement vector if the motion is rectilinear and there have been no changes in direction during the journey.

Speed and Velocity

Speed: The magnitude that describes how quickly the position of a moving object changes.

Average Velocity: This is a vector quantity; its direction and sense are the same as that of the displacement vector.

Acceleration

Acceleration: A quantity that describes how the velocity of a moving object changes.

Average Acceleration: This is a vector quantity with the same direction as the change in velocity (Δv).

Forces and Dynamics

Effects of Forces

  • Deformation of bodies.
  • Change in the state of motion of bodies.

Newton’s Principles of Dynamics

Key insights from Isaac Newton:

  • Forces are causes of change in motion, not motion itself; i.e., forces are related to acceleration, not velocity.
  • For forces to manifest, at least two bodies are required; namely, the concept of force is intimately linked to the interaction between two bodies.

Newton’s First Law: Inertia

A free body, i.e., a body upon which no net force acts (or the sum of the forces acting on it is zero), remains at rest or moves with uniform rectilinear motion.

Newton’s Second Law: F = ma

The total or resultant force (Ftotal) acting on a body is the product of its mass (m) and the acceleration (a) it communicates: Ftotal = m * a.

Newton’s Third Law: Action-Reaction

For every action a body exerts on another, there is an equal and opposite reaction exerted by the latter on the former: F1 = -F2.

Sliding Friction Forces

These are forces that oppose the sliding of one body over another.

Causes of Friction
  • Roughness of surfaces.
  • Cohesion and adhesion forces between atoms and molecules of the bodies in contact.
Characteristics of Friction Forces
  • They are parallel to the sliding surface.
  • They oppose the direction of motion (or impending motion) of the body.
  • They are directly proportional to the normal force (N). The normal force is perpendicular to the contact surface, and its value and direction are such that they keep the body on the surface.