Newton’s First Law of Motion

What Is Newton’s First Law of Motion?

Newton’s first law of motion states that

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 1st law of motion 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 will not stop or change its velocity until and unless some force acts upon it once more. The first law of motion is sometimes also known as the law of inertia.

There are two conditions on which the 1st law of motion is dependent:

• Objects at rest: When an object is at rest, velocity (v = 0) and acceleration (a = 0) are zero. Therefore, the object continues to be at rest.
• Objects in motion: When an object is in motion, velocity is not equal to zero (v ≠ 0), while acceleration (a = 0) is equal to zero. Therefore, the object will continue to be in motion with constant velocity and in the same direction.

What Is an External Force?

An external force is defined as the change in the mechanical energy that is either the kinetic energy or the potential energy in an object. These forces are caused by external agents. Examples of external forces are friction, normal force and air resistance.

Let Us Understand the First Law of Motion by an Example

Let us 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 us 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.

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

Thus, the first law of motion is confirmed again.

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

Newton’s First Law of Motion Example in Daily Life

Wearing a seat belt in a car while driving is an example of Newton’s 1st law of motion. If an accident occurs or brakes are applied to the car suddenly, the body will tend to continue its inertia and move forward, probably proving fatal. To prevent such accidents, seat belts are used, stopping your body from moving forward in inertia and avoiding danger.