# Law of inertia (inertial observer and inertial frames of reference)

• Vigorous
In summary, an inertial observer is an object with zero net force acting on it and moving with constant velocity. An inertial frame is a set of coordinate axes in which the particles' location and velocity can be measured relative to. The law of inertia applies to inertial observers, but in order to fully understand it, the forces acting on the particle must also be analyzed. This can be easily done with general relativity or Cartan's version of Newtonian physics. It is important to consider all of Newton's Laws together, as they work together to explain the behavior of objects. Additionally, Newton's 3rd Law helps determine what constitutes as a real force, which includes gravity in Newtonian physics.
Vigorous
I am trying to figure out what are inertial observer and inertial frames of reference. The law of inertia holds for inertial observers. Inertial observers are objects with zero net force acting on them, and move with constant velocity. Suppose we fix a set of coordinate axis in space, relative to that set of coordinate axis we can measure the particles location with time. Hence measure its velocity and know if it changes or not. But we still haven't tested the requirement that no forces are acting on the particle. so what I am trying to get at, is that an inertial observer is a particle analysed for the forces acting on it and if they sum to zero and the velocity is changing then the law of inertia does not hold. But why do we come to this conclusion and not say that the force analysis done on the particle is incomplete. Acceleration calls for a force.

This is somewhat difficult to do experimentally using Newtonian physics. However, with general relativity, or even with Cartan’s version of Newtonian physics, it becomes very straightforward:

An inertial object is one where an attached accelerometer (the 6 degree of freedom kind) reads 0.

An inertial frame is one where inertial objects have 0 coordinate acceleration.

Vigorous said:
The law of inertia holds for inertial observers.
It doesn't make sense to apply just one of Newtons Laws. The only make sense together.
Vigorous said:
But why do we come to this conclusion and not say that the force analysis done on the particle is incomplete.
Newtons 3rd Law tells you what qualifies as real forces, and in Newtonian physics that includes Gravity.

## 1. What is the law of inertia?

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

## 2. What is an inertial observer?

An inertial observer is an observer who is not accelerating or experiencing any net force. In other words, they are in a state of constant velocity or at rest.

## 3. How is an inertial frame of reference defined?

An inertial frame of reference is a coordinate system in which Newton's first law of motion holds true. This means that an object in motion will continue moving in a straight line at a constant speed unless acted upon by an external force.

## 4. What is the significance of inertial frames of reference?

Inertial frames of reference are important in understanding the laws of motion and how objects behave in the absence of external forces. They allow us to make accurate predictions and calculations about the motion of objects.

## 5. Can an inertial frame of reference be accelerated?

No, an inertial frame of reference cannot be accelerated. If it were to accelerate, then objects within that frame of reference would experience a change in their motion, violating Newton's first law of motion.

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