# What is inertia? What is its source?

• SinghRP
In summary: Inertia is mass that resists change in motion. Mach thought that it was due to the rest of the universe. General Relativity has not yet accommodated this idea.
SinghRP
What is inertia? What is its source? Is inertial mass mass? Do gravitational mass and inertial mass have the same sign?

Inertia is a term physicists don't use by itself all that often. The meaning of "inertia" is a bit ambiguous: Does it mean mass, linear momentum, angular momentum, mass*distance squared, or something else? Without context, it is a bit difficult to tell. What do you mean by "inertia"?

Physicists eschew the word for a simple reason: Why use an ambiguous term when there are perfectly good and much less ambiguous alternatives?

Well, I started this inertia topic. This one is not clear even today. I am working on it as a research topic.
Galileo, Eotovos, Newton, Mach, Enistein, Dicke addressed it. Inertia is resistance to acceleration. Mach thought, but could not prove it, that inertia is due to the rest of the mass in the universe. Einstein's principle of equivalence: inertial mass and gravitational mass are the measures of the same quantity. General Relativity has not yet accommodated it.
Well, maybe some one can shed some light.

Inertia: the resistance an object has to a change in its state of motion.

Are you familiar with what heat capacity is? Its the amount of energy it takes to raise the temperature of something by 1 degree.

Inertia would be quantitatively analysed as the amount of energy it takes to raise the speed of a mass by 1m/s . Something quite heavy would have a large value for inertia, just like water has a large value for heat capacity.

Also note that this works the other way around. It would take the same energy to slow it down as it would to speed up, and the same energy to heat up as it would to cool down.

Hamden and DH,

I agree with you. Inertia is not only with mechanical motion. One example is the one, DH, you cited.

In the mechanical world, I like to use momentum (including angular moentum) instead of motion, even though motion is by all means okay. Reason: orbital, vibrational and spin motions may not be included in motion when describing inertia. (At very low tepperatures you will need more heat to raise the temperature by 1 degree.)

Einstein: gravitational mass is numerically equal to inertial mass. Scenario: a body falling to the earth. As the gravitational mass is more, gravitational force increases; but then inertial mass is equally more, which increases the inertia or resistance. That's why two bodies with different masses fall with the same acceleration.

Let's continue this discussion. What's the source of inertia? Mach thought it was due to the rest of the universe. [The body does not resist, but the universe does?] What do you think?

## 1. What is inertia?

Inertia is the tendency of an object to resist changes in its state of motion. This means that an object at rest will remain at rest, and an object in motion will continue moving in the same direction and at the same speed, unless acted upon by an external force.

## 2. What causes inertia?

The source of inertia is mass. The more mass an object has, the more inertia it has. This is why larger objects require more force to start moving or to change their speed or direction.

## 3. How is inertia related to Newton's First Law of Motion?

Newton's First Law of Motion states that an object will remain at rest or in motion at a constant velocity unless acted upon by an external force. This is essentially describing the concept of inertia, as an object will only change its state of motion if an external force is applied to overcome its inertia.

## 4. Can inertia be measured?

Yes, inertia can be measured by calculating an object's mass. Mass is a measure of the amount of matter an object contains, and as mentioned before, the more mass an object has, the more inertia it has.

## 5. How does inertia affect objects in space?

In space, where there is no air resistance or other external forces, objects will continue moving in a straight line at a constant speed due to their inertia. This is why astronauts in space must use thrusters to change their direction or speed, as there is no air drag to slow them down.

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