# Newton's first law of motion and Inertial Systems

• rohit dutta
In summary, the book "An Introduction to Mechanics" by Kleppner and Kolenkow presents Newton's first law as an assertion that inertial frames exist. However, it also states that this law is part definition and part experimental, and that the existence of inertial frames is a statement about the physical world. This may lead to confusion as it is believed that inertial frames do not actually exist due to the curvature of space-time. The book only focuses on isolated bodies, which may leave an incomplete understanding of the first law. It is likely that the book is referring to frames that are inertial to first order, rather than true inertial frames.
rohit dutta
I have just begun my journey in Classical Mechanics with the book "An Introduction to Mechanics" by Kleppner and Kolenkow. I find myself stuck at Newton's first law. The book presents Newton's first law as follows:

"Newton's first law is an assertion that inertial frames exist."

The book also states:

"Newton's first law is part definition and part experimental. Isolated bodies move uniformly in inertial systems is by virtue of the definition of an inertial system. In contrast, the assertion that inertial systems exist is a statement about the physical world."

The above statements lead me to believe that inertial systems exist. However, it is believed that an inertial system does not exist. Does this belief not contradict the statements above? Do inertial frames really exist?

Also, in an inertial frame, not only isolated bodies but anybody experiencing a net zero force moves with constant velocity( or moves uniformly ). This is the first law that we all know. The book, however, focuses only on isolated bodies. Does this not leave us with an incomplete understanding of the first law?

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rohit dutta said:
The above statements lead me to believe that inertial systems exist. However, it is believed that an inertial system does not exist. Does this belief not contradict the statements above? Do inertial frames really exist?
Inertial frames do not exist because of the curvature of spacetime. But frames exist that are inertial to first order, and that is probably to what the text is referring.

andrewkirk said:
Inertial frames do not exist because of the curvature of spacetime. But frames exist that are inertial to first order, and that is probably to what the text is referring.

What has the curvature of space-time got to do with the existence of inertial frames? Also, what do you mean by frames are inertial to first order?

## What is Newton's first law of motion?

Newton's first law of motion, also known as the law of inertia, states that an object at rest will remain at rest and an object in motion will remain in motion with a constant velocity unless acted upon by an external force.

## What is an inertial system?

An inertial system is a reference frame in which Newton's first law of motion is true. This means that an object in motion will not experience any acceleration or a change in its velocity unless acted upon by an external force.

## How does Newton's first law relate to everyday life?

Newton's first law can be observed in many everyday situations. For example, when a car suddenly stops, objects inside the car will continue to move forward due to their inertia until they are acted upon by the seatbelt or airbag. Similarly, when a ball is thrown, it will continue to move in a straight line unless acted upon by the force of gravity or another external force.

## What is the difference between Newton's first law and the concept of inertia?

Newton's first law is a statement about the behavior of objects in motion, while inertia is a property of matter that describes an object's resistance to changes in its state of motion. In other words, Newton's first law explains how inertia affects an object's motion.

## Can Newton's first law be violated?

No, Newton's first law is a fundamental law of physics and has been observed to hold true in all situations. However, in certain cases, it may appear to be violated due to other forces acting on an object that are not initially considered.

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