Newton's First Law of Motion: Explained with Examples and Strategies for Competitive Exams

Newton’s First Law of Motion states that when an object is at rest, it stays at rest. But if an object is in motion at constant velocity, it will continue to move. Only an external, unbalanced force will stop its motion. 

Under Section 4.4 of the NCERT Physics textbook from CBSE, you will find the simplified description.

Newton’s First Law states that

“Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.”

Key Phrases in Newton’s First Law Simplified

“State of Rest”

When the law refers to a body being “in its state of rest,” it means that if something isn't moving, it will remain stationary unless something pushes or pulls it.

“Uniform Motion in a Straight Line”

This part means that if an object is already moving, and it's going at a constant speed in a straight line (which we call “uniform velocity”), it will keep doing that forever, as long as no external force acts on it.

So, if a body is moving with the uniform velocity, its acceleration is zero. Generally, we consider a horizontal motion.

You should review the formula for instantaneous velocity.

“Unless Compelled by Some External Force”

This phrase tells that the only way to change an object's state of motion is by applying a net external force.

• One key thing to remember is that an external force is a push or pull that is exerted on an object from outside its own boundaries.
• You also should focus on the word net. If multiple external forces are acting on an object, but they all cancel each other out, the net force is zero. And then the object's motion won't change, because it is not compelled to change its state by itself.

Sir Isaac Newton formalised the observations of Galileo Galilei (Law of Inertia) into the First Law of Motion in Philosophiæ Naturalis Principia Mathematica, 1687. The three laws of motion appear in this book.

Together, both Galileo and Newton could disprove the Aristotelian physics, that birthed natural philosophy in the ancient Greek times.

This principle was the first of the three Newton's Laws of Motion.  It corrected the earlier misunderstandings of force and motion through scientific experiments of Galileo and mathematical observations of Newton.

In fact, this Newtonian law is the definition of inertia, which describes a characteristic of a body that makes it resist any change to its state of motion.  

This scientific law of inertia led Einstein to develop his theory of relativity in the 20th century. 

Let's see how Newton's 1st Law of Motion applies to objects in rest or in uniform motion in the real world.

How Transportation Systems Work Based on Law of Inertia

When a train starts moving, passengers feel pushed backwards in the opposite direction. This is because of their inertia. For an object in motion, when it stops, they feel pushed forward.  

These experiences are direct applications of the First Law. They help us understand why we need seat belts, handrails, and other safety features in vehicles.

First Law of Newton in Sports and Athletics

Many sports phenomena can be explained through Newton's First Law. 

You can consider a hockey puck that slides on ice. It continues to move until friction and air resistance slow it down. Based on this law, the friction and air resistance make the puck becomes compelled to change.  

This Law Applies to Space Exploration Considering Air Resistance

In the vacuum of space, where friction and air resistance are negligible, this law becomes particularly evident. 

Spacecraft continue moving at the same velocity unless acted upon by rocket engines or gravitational forces. 

This principle of Newton helps astronauts and engineers know about orbital mechanics and space navigation.

Newton’s First Law is a special case of his Second Law of Motion, F = ma. That is, force equals mass multiplied by acceleration. 

when F = 0:

F_net = 0  

So, a = 0

This means the acceleration is zero. So velocity stays constant.

• Constant Velocity includes both “at rest” (v = 0) and “uniform motion” (v ≠ 0).
• Practice plotting velocity vs. time: When the net force is zero, the graph is a horizontal line.

Beyond your CBSE board Physics textbook, you need to be clear with some more related concepts when appearing for JEE Main. While you should get started with the NCERT Solutions for Laws of Motion to practice the main exercises, there are a few more aspects to consider for engineering entrances and your future studies.   

One is the relation between the mass of the object and inertia. Then come inertial and non-inertial frames of reference. Along with that, you would have to apply the concept in real-world multi-body systems and understand what types of constraints exist to prevent motion with constant or uniform velocity. 

In such cases, consider the following approaches.

Types of Inertia

There are three types of inertia to learn in Newton’s First Law. 

• Inertia of Rest: A stationary object resists being moved.
• Inertia of Motion: A moving object resists being stopped or slowed.
• Inertia of Direction: A moving object resists changes in its direction.

Mass and Inertia Relationship

All you have to remember is that the greater the mass, the greater the inertia. 

For instance, a heavy truck resists changes in motion far more than a bicycle.

Balanced and Unbalanced Forces

Among the core concepts of Newton’s first law are balanced and unbalanced forces. In balanced forces, it’s the equal forces that cancel out. That results in zero net force. In unbalanced forces, we have forces that don’t cancel each other. That causes acceleration.

Reference Frames and Inertial Systems

There are two. Inertial and non-inertial frames of reference. 

• Inertial reference frames tell us that objects with no net force acting on them remain at rest or move with constant velocity. 
• In non-inertial reference frames, also known as accelerating frames, Newton's First Law appears to be violated. Objects seem to accelerate without any apparent force acting on them.

Here are some essential approaches to this law of Newton that you should know when preparing for JEE Main papers. 

Learn About Multi-Body Systems

In JEE problems, multi-body objects often move together. There, you have to use the First Law of Newton to check if each body is in equilibrium or shares the system’s acceleration.

Know How to Treat Constraints

Objects such as strings and rods create motion limits. They force connected bodies to accelerate at the same rate. You must treat them as a single unit when solving.

Visualise Motion with Free-Body Diagrams (FBDs)

In JEE Main, many problems test whether you can correctly identify balanced and unbalanced forces. Free‑body diagrams (FBDs) help you achieve a few things. 

1. List all forces (gravity, normal, friction, applied).
2. Check if net force = 0 (balanced) or ≠ 0 (unbalanced).
3. Predict motion (constant velocity vs. acceleration).

You can also look into our simplified guide to Solving Problems in Mechanics aligning with the latest NCERT syllabus of 2025-26.