Inertia and Newton's Third Law of Motion

Inertia and Newton’s Third Law of Motion

Understanding Inertia

  • Inertia is the reluctance of an object to change its state of motion.
  • It is a property inherent to all material objects and is related to their mass.
  • An object with a greater mass (more inertia) is harder to start or stop moving than an object with less mass (less inertia).
  • An object at rest tends to stay at rest until acted upon by an external force.
  • An object in motion tends to stay in motion, maintaining its speed and direction (its vector), until acted upon by an external force.

Identifying the Third Law of Motion

  • Newton’s Third Law of Motion states: “To every action, there is an equal and opposite reaction”.
  • This law implies that all forces are interactions between different bodies, and thus, there is no such thing as a unidirectional force or a force that acts on only one body.
  • When body A exerts a force on body B, body B simultaneously exerts a force on body A with the same magnitude in the opposite direction.

Applying Newton’s Third Law of Motion

  • Newton’s Third Law applies everywhere and to all objects, regardless of their states of motion.
  • The forces described in Newton’s Third Law are sometimes also referred to as action-reaction force pairs.
  • Importantly, these paired forces always act on different bodies; one object experiences the action force, and the other experiences the reaction force.
  • An example is jumping off a small boat: As you push the boat backwards (action), the boat pushes you forwards (reaction).
  • Despite the forces being equal in magnitude and opposite in direction, the effect of the forces may not be similar because of differing masses and, thus, different amounts of inertia.

Differentiating between Mass and Weight

  • Mass is the amount of matter in an object and is a measure of its inertia. It is a scalar quantity and does not change with location.
  • Weight is the result of the gravitational force acting on an object’s mass. It is a vector quantity and changes if the strength of the gravitational field changes.

Identifying Units of Measurements

  • The unit of measurement for inertia is essentially kilograms (kg) as it directly relates to mass.
  • Newton’s Third Law deals with forces, which are measured in newtons (N).
  • Mass is measured in kilograms (kg) and weight in newtons (N).