Newton’s Laws of Motion


The laws of motion by Newton explain the movement of large bodies and their interaction. These laws also discuss properties like acceleration, force, velocity, the force of gravity, etc.

The first law which is also known as the law of Inertia deals with the forces acting on the body. A body remains in its state of rest or uniform motion provided no net force acts on it. Basically, inertia is the property of the body due to which it withstands any change at rest or motion.

The second law is concerned when a net force is acting on a body. The force produces acceleration in the body of mass m, the magnitude of this acceleration is directly proportional to force but inversely proportional to the mass of the body. a ∝ F and a ∝ 1/m.

The equation by putting constant become F= ma. Newton’s third law deals with the reaction of the body and states that to every reaction there is equal but an opposite reaction. In this article, the laws of Newton’s are explained.

An Introduction

Newton was one of the most influential researchers of all time. His ideas laid the foundation for modern physics. In “Principia”, Newton set out three simple rules of the universe.

Sir Isaac Newton’s three laws of motion describe the movement of huge bodies and how they interact. While Newton’s laws might seem obvious to us today, more than 3 centuries ago they were thought revolutionary.

In creating his three laws, Newton simplified his treatment of huge bodies by considering them to be mathematical points with no size or rotation. This allowed him to overlook factors such as friction, air resistance, temperature level, material properties, and so on, and focus on phenomena that can be explained entirely in regards to mass, length, and time.

Here are the few terms associated with these laws so you can understand the concept.


A force moves or tends to move stops or tends to stop the movement of a body. The force can likewise change the direction of the movement of a body.


Inertia of a body is its property due to which it withstands any change in its state of rest or movement.


Momentum of a body is the quantity of movement it has due to its mass and velocity.

Newton’s First Law of Motion


A body continues its state of rest or of uniform motion in a straight line provided no net force acts on it.


The very first law of motion deals with the bodies which are either at rest or moving with constant speed in a straight line. According to Newton’s first law of motion, a body at rest remains at rest supplied no net force acts upon it. This part of the law holds true as we observe that objects do not move by themselves unless some external force acts on them.

For example, a book pushing a table stays at rest as long as no net force acts on it. Likewise, a moving item does not stop moving by itself. A ball rolled on a rough ground stops earlier than that rolled on smooth ground. It is due to the fact that rough surface areas provide higher friction. If there would be no force to oppose the motion of a body then the moving body would never stop.

The net force is the resultant of all the forces acting upon a body.

Considering that Newton’s first law of motion deals with the inertial property of matter, therefore, Newton’s first law of motion is also referred to as the law of inertia.

Interesting Fact:

When a bus takes a sharp turn, passengers fall in the outward direction. It is due to inertia that they want to continue their motion in a straight line and thus fall outwards.


Newton’s Second Law of Motion:


When a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting upon the body and inversely proportional to its mass.


Newton’s 2nd law of movement deals with situations when a net force is acting upon a body.

Mathematical Form:

If a force produces a velocity a in a body of mass m, then we can mention mathematically that

According to the statement




So, by combining them


Putting ‘K’ as proportionality constant

F = K ma


The value of K in SI unit is 1 so we can say that

F = ma

SI Units:

SI system of force is the newton (N). According to Newton’s second law of movement:

One newton (1 N) is the force that produces an acceleration of 1 ms-2 in a body of mass of 1 kg.

So, a force of one newton can be expressed as


Newton’s Third Law of Motion


To every action, there is an equal but opposite reaction.


Newton’s third law of motion deals with the response of a body when a force act upon it. Let a body A exerts a force on another body B, the body B reacts against this force and applies a force on body A. The force applied by body A on B is the action force whereas the force applied by body B on A is called the reaction force.

According to this law, the action is always accompanied by a reaction force and the two forces need to always be equivalent and opposite.


Think about a book resting on a table. The weight of the book is acting upon the table in the downward direction. This is the action. The reaction of the table acts on the book in an upward direction.


Consider another example. Take an air-filled balloon. When the balloon is released, the air inside it rushes out and the balloon moves upward. In this example, the action is by the balloon that presses the air out of it when set free. The reaction of the air which leaves out from the balloon acts on the balloon. It is because of this reaction of the leaving air that moves the balloon forward.