Unit 2.4: Newton’s First Law

Introduction

Newton’s First Law, also known as the Law of Inertia, describes the behavior of objects when no net force acts upon them. It states that an object at rest stays at rest, and an object in motion continues in motion with the same velocity unless acted upon by an external force. This principle explains why objects resist changes in motion and is fundamental to understanding forces in physics.

Key Concepts

Inertia: The tendency of an object to resist changes in motion. More mass means more inertia.
Net Force (\(\sum F\)): If the net force on an object is zero, its velocity does not change.
Balanced vs. Unbalanced Forces: If all forces cancel out, the object remains in its current state of motion.
Applications: Seat belts in cars, objects sliding to a stop due to friction, and astronauts floating in space.

Tip: If an object is moving at constant velocity or at rest, the net force is zero. If its velocity changes, there must be a net force acting on it.

Common Misconceptions

"Objects need a force to keep moving." In reality, an object moves indefinitely in the absence of external forces (e.g., friction).
"Heavier objects fall faster." All objects experience the same acceleration due to gravity in a vacuum.
"If an object is at rest, no forces are acting on it." Objects at rest may still experience forces, but they balance out (e.g., a book on a table).

Free-Body Diagrams and Newton’s First Law

Free-body diagrams help visualize forces acting on an object. If all forces sum to zero (\(\sum F = 0\)), then the object is either at rest or moving at constant velocity.

Draw arrows for all forces acting on the object.
Identify pairs of balanced forces (e.g., gravity and normal force for an object on a table).
Determine whether the forces sum to zero or if there is a net force.

Exam Tip: If an object is moving at a constant velocity, explicitly state that the forces are balanced when explaining your reasoning.

Practice Activities

Activity 1: Identifying Forces at Equilibrium

Identify the forces acting on the following objects and determine if they are balanced:

A book resting on a table
A person sitting in a car moving at a constant speed
A skydiver falling at terminal velocity

Activity 2: Newton’s First Law in Real Life

Explain how Newton’s First Law applies to each of the following:

A passenger lurching forward when a car suddenly stops
An astronaut floating in space
A hockey puck gliding across an ice rink

Summary & Exam Preparation Tips

Newton’s First Law is a fundamental concept in physics. Key takeaways:

Objects resist changes in motion due to inertia.
If net force = 0, the object maintains its current state of motion.
Objects in motion do not require a force to keep moving, only to change motion.
Use free-body diagrams to determine whether forces are balanced or unbalanced.

Understanding this law helps with problem-solving and real-world applications like seat belts, space travel, and friction effects.