Newton's First Law of Motion: Definition, Formula and Real-Life Examples

Newton's first law of motion explains why a book stays on a table and why a moving bicycle keeps rolling for some distance even after you stop pedalling. It tells us how force affects the motion of objects around us. Have you ever noticed that passengers move forward when a bus stops suddenly? The answer lies in this simple law of physics. This article gives a detailed explanation of Newton's first law of motion, its formula, examples, and applications in everyday life.

Table of Contents

• What is Newton's First Law of Motion
• Mathematical Formula for Newton's First Law of Motion
• Important Conditions of Newton's First Law of Motion
• Newton's First Law of Motion Examples in Daily Life
• Common Misconceptions About Newton's First Law of Motion

What is Newton's First Law of Motion

Newton's first law of motion states,

“An object remains at rest or continues moving with the same speed in a straight line unless an external force acts on it.”

In simple words, an object does not change its state of motion on its own. This law is also called the law of inertia because it explains the property of inertia in objects.

Statement of the Law

• A stationary object remains stationary.

• A moving object keeps moving with constant speed and direction.

• A change happens only when an external force acts on the object.

Interestingly! This law explains many events that we observe in our daily life. So let's try to understand this with a real-life example. 

A School Bag on a Bus Seat

Imagine you place your school bag on the seat of a bus that is not moving.

At this moment, the bag is at rest. The force of gravity pulls the bag downward, and the seat pushes it upward with an equal force. Since these two forces balance each other, there is no unbalanced external force acting on the bag.

Therefore, the bag remains at rest, which follows Newton's first law of motion.

Now, suppose the bus suddenly starts moving forward. You may notice that the bag slides backward on the seat. But why is this happening?

The bag wants to remain in its state of rest due to inertia. Since the bus moves forward suddenly, the bag resists this change in motion and appears to move backward relative to the bus.

Similarly, if the moving bus suddenly stops, the bag moves forward because it tends to keep moving with the bus's previous speed.

Thus, the behavior of the school bag confirms Newton's first law of motion.

Mathematical Formula for Newton's First Law of Motion 

The law itself is a statement, but it can be represented using the idea of net force.

If the net external force is zero, then acceleration is also zero.

Fnet=0

This means:

• The object remains at rest, or
• The object moves with constant velocity.

The fact is that if there is no unbalanced force, the object's motion will not change.

Important Conditions of Newton's First Law of Motion

Newton's first law of motion works when the net external force acting on an object is zero.

In physics, this condition is written as,

∑F=0

This means that all the forces acting on the object are balanced. As a result, the object's acceleration becomes zero.

a = 0

Now, how does all this come together? Let's find out.

Objects at Rest (Static Equilibrium)

In this case,

• Velocity, v = 0
• Acceleration, a = 0

The object remains stationary because all the forces acting on it cancel each other.

Example: A book placed on a table stays at rest. The downward pull of gravity and the upward push of the table are equal, so there is no unbalanced force.

The object will start moving only if an external force acts on it.

Objects in Motion (Dynamic Equilibrium)

In this case,

• Velocity, v≠0 and remains constant.
• Acceleration, a = 0

The object keeps moving with the same speed in the same direction because there is no unbalanced force to change its motion.

Example: A hockey puck sliding on smooth ice keeps moving for a long distance because there is very little friction.

Interestingly! If there were no friction or air resistance, the object would continue moving forever.

Newton's First Law of Motion Examples in Daily Life

Newton's first law of motion can be seen in many situations around us. The fact is that objects always try to keep their current state of motion unless an external force changes it.

• Water Spilling from a Container:Imagine carrying a bowl filled with water and suddenly starting, stopping, or turning. The water spills because it resists the change in motion and tends to continue doing what it was doing before.
• Coffee Spilling in a Moving Car:Have you ever noticed coffee spilling when a car suddenly starts or stops? When the car moves forward, the coffee tries to remain at rest. Similarly, when the car stops, the coffee keeps moving forward due to inertia.
• Seat Belts in Cars: When a moving car stops suddenly, the passengers continue moving forward because their bodies want to keep moving. Seat belts provide an external force that stops the passengers and prevents injuries.
• Tightening a Hammer Head:A loose hammer head can be tightened by striking the bottom of the handle on a hard surface. The handle stops suddenly, but the heavy hammer head continues moving due to inertia and fits tightly onto the handle.
• Removing Ketchup from a Bottle:Have you heard about shaking a ketchup bottle upside down? When the bottle is moved downward quickly and stopped suddenly, the ketchup keeps moving and comes out of the bottle.
• Headrests in Cars:Headrests are provided to protect passengers during rear-end collisions. When the car moves forward suddenly, the head tends to remain at rest, which can cause neck injuries. The headrest supports the head and reduces the risk of whiplash.
• Falling Forward from a Skateboard:Imagine riding a skateboard and hitting a stone. The skateboard stops immediately, but your body keeps moving forward because of inertia, causing you to fall.
• Feeling Heavier in an Elevator:When an elevator moving downward stops suddenly, blood tends to continue moving downward due to inertia. This creates a strange feeling in your body for a moment.

Common Misconceptions About Newton's First Law of Motion

Newton's first law of motion states that an object at rest stays at rest, and an object in motion keeps moving with the same speed and direction unless an unbalanced external force acts on it.

Even though this law is simple, many people have some wrong ideas about it. Let's clear them one by one.

A Moving Object Needs a Continuous Force to Keep Moving

This is not true. A force is needed only to change the speed or direction of an object, not to keep it moving.

For example: A football rolling on the ground stops because friction acts on it. If there were no friction, it would keep moving.

No Force Means No Motion

Many people think that if no force acts on an object, it cannot move. The fact is that zero net force means zero change in motion. An object can:

• Stay at rest, or
• Move with a constant speed in a straight line.

For example: A spacecraft in space can continue moving for a long time because there is very little friction.

Objects Naturally Come to Rest on Their Own

This is another common misconception. Objects do not stop by themselves. They stop because external forces such as friction or air resistance slow them down.

For example: A bicycle eventually stops because of friction between the tyres and the road.

Inertia is a Force

No, inertia is not a force. Inertia is a property of matter that makes an object resist changes in its state of rest or motion.

For Example: A loaded shopping cart is harder to push than an empty one because it has more inertia.

An Object at Rest Has No Forces Acting on It

This is not correct. A stationary object can have several forces acting on it, but these forces balance each other.

For example: A book lying on a table experiences:

• The gravitational force pulling it downward.
• The upward force of the table pushing it upward.

Since these forces are equal, the book remains at rest.

Heavier Objects Fall Faster Because of Inertia

This idea is also incorrect. Inertia affects resistance to changes in motion, but it does not decide how fast objects fall. In the absence of air resistance, all objects fall at the same rate.

For example: On the Moon, a feather and a hammer fall together.

Why Do We Get These Ideas Wrong?

Now you may ask, why do these misconceptions happen? The answer is friction.

In our daily life, almost every moving object eventually slows down because of friction or air resistance. Because of this, it is easy to think that objects stop because they "run out of force."

Interestingly, if friction did not exist, many objects around us would keep moving for a very long time.

Newton's first law of motion states that an object remains at rest or keeps moving with the same speed and direction unless an external force acts on it. We also learned about inertia, the law's formula, and its examples and applications in daily life. This law helps us explain many things that happen around us every day.