Short Answer 
Newton’s First Law, or the law of inertia, states that an object remains at rest or in constant motion unless acted upon by a net external force. This principle underscores how objects resist changes in their state of motion and demonstrates its relevance through everyday examples like a person lurching forward in a car when it accelerates or stops.
Step 1: Understanding Newton’s First Law
The first law of motion, known as the law of inertia, establishes that an object will remain in its current state (either at rest or moving) unless acted upon by an external net force. This means that for any physical change in motion to occur, a force must be applied. It’s crucial to grasp that both stationary and moving objects are affected by this principle.
Step 2: Implications of the Law
Newton’s first law conveys two key implications regarding motion:
- An object at rest stays at rest until a force causes it to move.
- An object in motion continues to move at a constant speed and in a straight line unless a net force intervenes.
This law highlights the importance of external forces in changing an object’s state, emphasizing that inertia resists changes to motion.
Step 3: Real-World Example
A practical illustration of the law of inertia can be seen in everyday situations, such as when a person is seated in a car with a seatbelt. As the car accelerates or stops, the person tends to lurch forward due to their body’s inertia, which resists the change in motion. This demonstrates that without the seatbelt (the external force), the person’s motion would remain unchanged, highlighting the law’s application in both static and dynamic scenarios.