Newton's First Law of Motion is all about inertia - an object's resistance to change in motion. It states that objects at rest stay at rest, and objects in motion stay in motion, unless acted on by an unbalanced force.

This law sets the stage for understanding motion and forces. It explains why objects behave as they do in everyday situations, from a book sitting on a table to a car continuing to move after you take your foot off the gas pedal.

Newton's First Law of Motion: Inertia

Newton's first law and inertia

• States an object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted upon by an unbalanced force
• Known as the law of inertia
• Inertia is the tendency of an object to resist changes in its state of motion
  • Objects with more mass have greater inertia require more force to change their motion (bowling ball vs. tennis ball)
• Inertia causes an object to continue in its current state of motion unless acted upon by an unbalanced force as described by Newton's first law
  • A book resting on a table will remain at rest until pushed
  • A rolling ball will continue rolling at the same speed and direction until friction or another force slows it down

Friction's impact on motion

• Friction opposes the relative motion between two surfaces in contact
  • Always acts in the direction opposite to the motion or attempted motion of an object
• Types of friction:
  • Static friction prevents an object from starting to move (a heavy box on the floor)
  • Kinetic friction opposes the motion of an object once it is moving (sliding a book across a table)
• Friction can cause an object to slow down, stop, or change direction
  • Without an unbalanced force to overcome friction, an object in motion will eventually come to rest due to the opposing force of friction (a sliding hockey puck eventually stops)
• When friction is the only force acting on an object, it is an unbalanced force that changes the object's state of motion as per Newton's first law
  • A coasting bicycle will slow down and stop because of friction unless the rider pedals to maintain speed

Systems and external forces

• A system is a collection of objects that interact with each other
  • An open system can exchange energy and matter with its surroundings (a pot of boiling water)
  • A closed system cannot exchange matter with its surroundings but can exchange energy (a sealed thermos)
• External forces originate outside the system being analyzed
  • Examples include gravity, friction, tension, and applied forces
• To identify external forces:
  1. Define the system and its boundaries
  2. Identify all forces acting on the objects within the system
  3. Determine which forces are internal (originating from within the system) and which are external (originating from outside the system)
• External forces are the unbalanced forces that can change an object's state of motion
  • If the net external force on an object is zero, the object will maintain its state of motion as described by Newton's first law (a satellite orbiting Earth at a constant speed)

Motion and Forces

• Velocity is the rate of change of position and includes both speed and direction
• Acceleration occurs when there is a change in velocity, either in magnitude or direction
• A force is any interaction that, when unopposed, will change the motion of an object
• Newton's laws of motion describe the relationship between forces and motion
  • Newton's First Law of Motion (inertia) is part of this broader set of laws
• Equilibrium is achieved when the net force acting on an object is zero, resulting in no acceleration