Newton’s First Law of Motion: We all know that if an object is at rest with respect to us, then an external agent is always required to change its state of rest. What is this external agent? How it changes the state of rest of an object? Why does an object moving along a straight line eventually comes to rest on its own? The force acting on the object causes it to change its state of rest or motion. Force is the external agent that can change the speed and direction of the motion of an object.

What is Newton’s First Law of Motion?

According to Newton’s first law of motion, a body does not change its state of rest or uniform motion unless acted upon by some unbalanced force. The body will remain in the same state of rest or motion along the same direction unless a force is involved in changing the body’s inertial state of rest or motion. In other words, it states that a body will continue in its state of rest or of uniform motion in a straight line unless compelled by some applied unbalanced force to change its state of rest or uniform motion.

Newton’s First Law of Motion – Galileo’s Idea on Force and Motion

Galileo observed the motion of objects on an inclined plane and made some deductions that lead to the discovery of Newton’s laws of motion.

1. When a marble rolls down a frictionless inclined plane, it gains velocity due to the unbalanced force of gravity. It attains a definite velocity by the time it reaches the bottom.
2. The velocity of the marble decreases as it climbs up on the frictionless inclined plane.
3. In figure (c), when the marble rolls down the frictionless inclined plane from the left-hand side, its velocity increase, but when it climbs up on the frictionless inclined plane on the right-hand side, it decelerates till it reaches the same height. At this maximum height, the velocity of the marble becomes zero. When the slope of the plane on the right-hand side is reduced, then the marble will move farther away on the inclined plane. If the slope of the right-hand side plane is reduced to zero, the marble will continue moving forever, trying to reach the same height that it was released from. The unbalanced force acting on the marble, in this case, will be zero. From this, we can conclude that an unbalanced force is required to change the motion of the marble. It is difficult to achieve zero unbalanced force because there exists a force of friction between the point of contact of the two objects, which are in relative motion. It always opposes the motion of an object on a surface.

Newton’s First Law of Motion – Statement

An object remains in its state of rest or of uniform motion along a straight line unless compelled to change that state by an applied unbalanced force.

According to Newton’s first law, all objects oppose a change in their state of rest or motion, and the tendency of an object to be in their state of rest or motion with the same velocity is called inertia. Hence, Newton’s first law of motion is also known as the law of inertia.

What is Inertia?

The word inertia means resistance to change. Inertia is the natural tendency or property of an object by virtue of which it resists any change in its state of rest or uniform motion along a straight line. Inertia is measured by the mass of an object. The heavier the object, the larger will be its inertia. For example, the inertia of a box having a mass \({\rm{6}}\,{\mkern 1mu} {\rm{kg}}\) will be more than the inertia of a box having a mass \({\rm{2}}\,{\mkern 1mu} {\rm{kg}}{\rm{.}}\) The greater the inertia of a body, the greater is the force required to change the state of rest or motion of an object.

In the above figure, the mass of the box is less, so its inertia is less. Therefore, the force applied by the boy to move the box is less. But in the second figure, the mass of the elephant is more, so it requires a lot of force to make the elephant move because it has greater inertia.

Types of Inertia

There are three types of inertia:

1. Inertia of Rest: The tendency of an object to be in its state of rest and resist any change in this state when some external unbalanced force is applied to it is called the inertia of rest.
2. Inertia of Motion: The tendency of an object to be in its state of motion and resist any change in this state when some external unbalanced force is applied to it is called the inertia of motion.
3. Inertia of Direction: The tendency of an object to resist any change in its direction of motion unless some external unbalance force is applied to it is called the inertia of direction.

Newton’s First Law of Motion – Examples

There are many activities in our daily life that are examples of Newton’s first law of motion:

1. When a vehicle suddenly starts moving, the passengers lean backwards. This happens because their feet which are in contact with the vehicle, move with the vehicle, but their upper bodies resist the change in the state of rest.

2. When a card placed over the mouth of a tumbler is flicked horizontally, the card flies away, but the coin kept on the card falls into the tumbler due to the inertia of rest.

3. When a running bike stops suddenly, the driver leans forward because of the inertia of motion. The lower body of the driver, being in contact with the bike, immediately comes to rest, but his upper body remains in the state of motion.

4. Passengers sitting in a bus that is moving along a straight road tend to fall sideways when the bus suddenly takes a sharp turn because of the inertia of direction as the passengers try to maintain their direction of motion.

Application of Newton’s First Law of Motion

A driver is advised to wear a seat belt while driving a vehicle because if an accident happens or if sudden brakes are applied, then the forward motion of the driver due to inertia can be slowed down. It will reduce the risk of injury by providing a balanced force in the opposite direction.

Importance of Newton’s First Law of Motion

Newton’s first law of motion gives an understanding of the inertial state of an object. It relates the motion of an object with force. Many of our daily life activity is based on Newton’s three laws of motion. According to Newton’s first law of motion or law of inertia, an external force is needed to change the speed and direction of the motion of an object. A net force greater than zero must be applied to change the speed and direction of motion.

From this article, we can conclude that the state of rest or motion of an object is governed by Newton’s first law of motion. It states that an object will continue in its state of rest or uniform motion in a straight line unless compelled by some applied unbalanced force to change its inertial state of rest or motion.

Newton’s First Law of Motion – FAQs

Q.3. What is the definition of force according to Newton’s first law of motion?
Ans: According to Newton’s first law of motion, force is an external cause that tends to change the state of rest or the state of motion of a moving object.

Q.4. State Newton’s first law of motion.
Ans: Newton’s first law states that an object remains in its state of rest or of uniform motion along a straight line unless compelled to change that state by an applied unbalanced force.

Q.5. How long will an object remain at rest according to Newton’s first law of motion?
Ans: According to Newton’s first law of motion, an object remains at rest until a net external force greater than zero acts upon it.