Newton's Laws of Motion - First Law


Sir Isaac Newton published three laws in \( 17^{th} \) century. In this article, we are going to talk about Newton’s first law. This law laid emphasis on the motion of the object and the force acting on it. In other words, it deals with the motion of an object and its relation to force.


“A body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.”

Putting Newton’s first law in simple words, a body will not start moving until and unless an external force acts on it. Once it is set in motion,it won’t stop or change its velocity until and unless some force acts upon it once more.

Let us understand Newton’s First Law of Motion by an example:

Let’s take a block on a smooth surface. By smooth, we mean that there is no friction acting on the surface. The block is at rest, that is, it is not moving.

Now, let’s examine the forces acting on the block. The only forces acting on the block are the force of gravity and the normal reaction by the surface. There is no force acting on it in the horizontal direction. Since the forces in the vertical direction are equal to each other in magnitude, they cancel each other out, and hence there is no external force on the block. Since this block is at rest, we can say that it confirms Newton’s first law of Motion.

Newton’s First Law of Motion

Now, if we apply a constant force F on the block in horizontal direction, it will start moving with some constant acceleration, in the direction of the applied force.

Newton’s First Law of Motion

Thus, Newton’s first law is confirmed again.

Note: Newton’s laws are valid only in inertial frames of reference.

Newton’s First Law of Motion Examples in Daily Life:

Wearing a seat belt in a car while driving is an example of Newton’s first law of motion. If an accident occurs, or if brakes are applied to the car suddenly, the tendency of the body will be to continue its inertia and move forward, probably proving fatal. To prevent such accidents, seat belts are used which stops your body moving forward in inertia avoiding danger.

FAQ on Newton’s First Law of Motion:

Let’s take a look at a few FAQ’s related to the topic.

1. What are the major concepts involved in the topic?

The major concepts involves: Frame of reference, Newton’s First law of motion (Law of Inertia), Newton’s Second law of motion, Newton’s Third law of motion (For every action there is an equal and opposite reaction) and constraint equations.

2. What are the characteristics of Normal Force?

Normal force is an electromagnetic force and always acts perpendicular to the surface of contact. For example,

3. What do we understand by Free Body Diagram?

A free body diagram represents all the forces acting on the body. For example,

Free Body Diagram

So, the FBD for sphere B will be as shown below,

Free Body Diagram

4. What are steps to solve a problem on Newton’s laws of motion?

Let’s take the following example,

Newton's First Law Examples

The wedge is fixed and we need to find the acceleration of the block of mass m along the incline.

Step 1: Draw the F.B.D of the block,

Free Body Diagram

Step 2: Writing the force equation taking our own axis, in this case, we take two axes one along the plane and the other perpendicular to it.

Fincline = \(mg~ sin45^{\circ}\)

VFNormal = \(N ~–~ mg~ cos45^{\circ}\)

Step3: Calculating acceleration using second law of motion

\(mg ~sin45^{\circ}\)  = \(ma\)

  • \(a\) = \(g~ sin45^{\circ}\)

5. What is constraint equation?

  When the motion of one body is governed by another or in other words when the motion of two or more bodies is interlinked. The equation governing such motion is known as constraint equation. For example:

Free Body Diagram

If we draw the FBD and write equations we will get,

\(M_1.g ~–~ T\) = \(M_1.a_1\)

\(2T – M_2.g\) = \(M_2.a_2\)

We can see there are two equations and three unknowns: \(a_1\), \(a_2\) and \(T\). So we need one more equation and that will be the constraint equation.

Stay tuned with Byju’s to know more about the Newton’s Laws of motion, newton’s second law and much more.

Practise This Question

A : A cloth covers a table, some dishes are kept on it. The cloth can be pulled out without dislodging the dishes from the table.
R : For every action there is equal and opposite reaction