Newton’s First Law of Motion – Definition and Applications

Newton’s First Law of Motion: Newton’s First Law of Motion asserts that an object maintains its state of rest or motion unless or until an external force acts upon it. It is also known as the law of inertia. In 1687, Mathematician and Physicist Sir Issac Newton discovered the First Law of Motion. The laws of motion are described in one of his books, Philosophiae Naturalis Principia Mathematica, also known as the Principia.

Newton formulated these laws of motion based on Galileo’s experiment. For example, a momentary jerk is felt by passengers sitting inside a bus when the driver abruptly applies the breaks. This can be attributed to the law of inertia. As a result of the law of inertia, our body maintains a state of motion even after the bus stops, thereby pushing us in the forward direction. Read this article to learn about Newton’s First Law of Motion in detail.

What is Newton’s First Law of Motion?

Newton’s First Law of Motion consists of three parts:

1. If no external force is applied, then a body at rest continues to be at rest. This is evident from observing our day-to-day life in which an object kept at a place does not move from its place until and unless it is moved after applying some force.

2. If no external force is applied, then a body in uniform motion continues to move uniformly. This can be explained by allowing a ball to roll on a smooth surface like a mirror and roll another ball on a rough surface like a floor. Both the balls will travel different distances based on the frictional force acting on them. The lesser the frictional force the more will be the distance covered by the ball as in the case of a ball on a mirror. If no frictional force acts on such a ball, then the ball will keep rolling infinitely. Thus, a body in uniform motion continues to move uniformly.

3. If no external force is applied, then a body cannot change its direction of motion; that is, it will continue to move along the same straight line. This can be seen when we take a turn while moving in a bus, we must put efforts to go along the bus and seated at the same position on our seats else we will be thrown sideways.

Thus, the first law of motion gives information about the situations when no external force acts on a body at rest or in uniform motion.

What Are the Laws of Motion?

First law of motion is one among Newton’s three laws of motion. Newton further investigated Galileo’s experiment and formulated three laws of motion. 

Galileo studied the motion of objects on an inclined plane. He observed that,

1. Objects moving down a smooth inclined plane accelerates.

2. Objects moving up a smooth inclined plane retard.

3. Objects moving on a frictionless horizontal plane move with a constant velocity. They neither have acceleration nor have retardation.

By taking two inclined planes with some slope, Galileo also observed that,

4. In an ideal situation where there is no friction, and the ball is rolled between two inclined planes with the same slope. The final height of the ball is the same as the initial height of the ball.

5. When the slope of the second smooth inclined plane is reduced, then the ball still reaches the same height after travelling a larger distance.

6. When the slope of the second smooth inclined plane is reduced to zero, then the ball travels an infinite distance and never ceases as it never reaches the same height.
From these observations, Galileo concluded that the state of rest and the state of motion with constant velocity are equivalent. In both cases, no net force is acting on the body. Thus, Newton formulated these laws of motion based on Galileo’s experiment.

Learn About Newton’s Law of Motion and Its Real-Life Applications

The three laws of motion as given by Newton are:

1. First Law of Motion: According to this law, a body continues to be in its state of rest or uniform motion along a straight line unless it is acted upon by some net unbalanced external force to change the state.
2. Second Law of Motion: According to this law, the rate of change of momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the force applied.
3. Third Law of Motion: According to this law, to every action, there is always an equal and opposite reaction.

Law of Inertia

Newton’s first law of motion defines inertia. Inertia is the inability of a body to change by itself its state of rest or state of uniform motion along a straight line.

This can be well understood by taking a small stone and a big stone. Heavier the stone, it becomes difficult for us to bring it into motion or stop its motion or even change its direction by hitting it with some object. Thus, a body tries to oppose any external force that tries to change its state of rest or state of uniform motion along a straight line.

Newton’s first law of motion also describes about the state of the body when no external force acts on it. In other words, it also talks about the ability of the body to maintain its state in the absence of any external force. Hence, Newton’s first law of motion defines inertia and is rightly called the law of inertia.

First Law of Motion Describes Force

First law of motion states that a body at rest continues to be at rest and a body in uniform motion continues to be in uniform motion along a straight line. That means, a force can change the state of rest of a body or can change the state of uniform motion of a body. So, first law of motion describes force which is as under:

Force is an external effort in the form of a push or pull which moves or tries to move a body at rest; stops or tries to stop a body in motion; changes or tries to change the direction of motion of a body.

Applications of First Law of Motion

First law of motion helps us in understanding the reason behind the behaviour of objects around us. Some of them are as mentioned below:

1. When we beat a carpet, dust comes out of it. This is because when we beat a carpet, the carpet comes into motion, but the dust tends to stay in its state of rest and thus comes out of the carpet.

2. We would have seen the magic trick in which the coin kept on a card over a glass just falls inside the glass when the card is flicked to move away. This is because the card comes into motion by the action of the force, but the coin retains its state of rest and hence falls into the glass when the card below it is flicked.

3. We often would have experienced being pushed backwards when the bus we are seated in starts to move. This is because when the bus comes into motion, the lower part of our body in contact with the seat of the bus comes into motion along with the bus, but the upper part of our body tends to retain its state of rest and hence gets a jerk backwards.

4. Similarly, when the bus we are seated in suddenly stops, we get pushed forward. This is because when the bus comes to rest from motion, the lower part of our body in contact with the seat of the bus comes to rest along with the bus, but the upper part of our body tends to retain its state of motion and hence gets a jerk forward.

5. Same is in the case when the bus we are seated in takes a steep curve. While taking a curve, the bus must change its direction, so we get pushed towards the opposite side. If the bus changes its direction towards the left, we get pushed towards the right and vice versa. This is because when the bus changes its direction, the lower part of our body in contact with the seat of the bus changes its direction along with the bus, but the upper part of our body tends to retain its motion along the straight line and hence tends to maintain the same direction and gets pushed in the opposite direction.

FAQs About Newton’s First Law of Motion

Let’s look at some of the commonly asked questions about Newton’s first law of motion:

Q.1. What are the types of inertia?
Ans: There are three types of inertia. They are inertia of rest, inertia of motion and inertia of direction.

Q.2. What is Newton’s first law of motion?
Ans: Newton’s First Law of Motion asserts that an object maintains its state of rest or motion unless or until an external force acts upon it. It is also known as the law of inertia.

Q.3. Who discovered laws of motion?
Ans: In 1687, Mathematician and Physicist Sir Issac Newton discovered the First Law of Motion. The laws of motion are described in one of his books, Philosophiae Naturalis Principia Mathematica, also known as the Principia.

Q.4. What are the three laws of motion?
Ans: First Law of Motion: According to this law, a body continues to be in its state of rest or uniform motion along a straight line unless it is acted upon by some net unbalanced external force to change the state.
Second Law of Motion: According to this law, the rate of change of momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the force applied.
Third Law of Motion: According to this law, to every action, there is always an equal and opposite reaction.

Q.5. What is inertia?
Ans: Inertia is the tendency of a body to remain in its state of rest or of uniform motion along a straight line.

Q.6. What is the other name of first law of motion?
Ans: The other name of first law of motion is law of inertia as it defines inertia.

Q.7. Which law of motion measures force?
Ans: Second law of motion measures force. Force can be calculated by taking the product of the mass of the body and acceleration produced in it by the force.

Q.8. Which physical quantity is described by first law of motion?
Ans: First law of motion describes force. It describes that a force is an external effort that may bring a body at rest into motion or may change the speed or direction of the moving body or both or even may stop a moving body.

We hope you find this article on First Law of Motion helpful. In case of any queries, you can reach back to us in the comments section, and we will try to solve them.