# What is Inertia?

• toffeecat25

#### toffeecat25

what is Inertia??

inertia is the state of an object at rest and a body in motion will move at constant velocity unless influenced by an outside force...

Hi, what you said is correct.
Inertia mainly refers to classical physics (in quantum physics it may be sometimes connected to momentum). A mass will remain in rest until it is affected by external forces. So that mass has more inertia. Suppose you travel in train..although you are moving you are at rest and therefore have some inertia to resist the travel...and depends on your mass.

Both of those seem very cumbersome to me. Why not just say inertia is a measure of the ability to resist acceleration?

inertia is the state of an object at rest and a body in motion will move at constant velocity unless influenced by an outside force...

To paraphrase Newton's 1st law in modern language, "To the extent the net force acting on a body is zero, the body's velocity remains unchanged." In a sense, Newton is defining the concept of inertial reference frames (IRFs) with his first law. (If a body's velocity varies when the net force on it is zero, then the body is not being observed from an IRF.)