# Reason behind Inertia

Has any physicist work on the reason behind Inertia? Why do we have inertia only during acceleration? No inertia during constant velocity.

• This eludes to the question of what is momentum. – John Alexiou Mar 6 '16 at 22:34

## 2 Answers

You have inertia all the time. Physics has advanced since Newton.

Now we know that an object has energy and momentum and that they combine together to give a 4d energy-momentum vector with four components. And mass is the length of that vector.

When objects have relative velocities that are small compared to the speed of light, those vectors point in almost the same direction so the sum of the lengths is really close to the length of the sum. And that's why two bowling balls have a mass that is really close to the sum of the mass of each.

We've also learned that mass isn't the source of gravity, that the stress-energy tensor is and that empty space itself can be curved in its own, even far from stress-energy.

We've also learned that you don't have to have mass to be affected by gravity.

And we learned that forces are really about exchanges of energy and momentum and so you can experience a force by exchanging energy and momentum with a field, even a field with zero mass (and that the squared lengths of the energy momentum can be zero even when the vector is nonzero).

So we've learned a lot.

• 'We've also learned that you don't have to have mass to be affected by gravity.' what???? where i should read about it? – Anni Mar 6 '16 at 8:52
• @Anni You can read it everywhere. Light doesn't have mass and yet it's trapped by a black hole. So it's probably something you've heard before, I'm just making a big deal about the things you need to unlearn. For instance in my two most recent answer (physics.stackexchange.com/a/241765) I point out that you think things fall because of gravity but really the inertial frames are the freefall frames and the earth is pushing you upwards when you stand on the surface because the pressure of the ground on your feet exerts a force on you. So light effectively "falls" too. – Timaeus Mar 6 '16 at 9:08
• You dis not answer my question in a direct manner. Shall I read energy stress tensors? Why does mass not resist velocity but only acceleration? I think no physicist could explain it so far. – Ahmet Gazi Mar 6 '16 at 17:15
• @AhmetGazi You asked why inertia only exists sometimes. The answer is that inertia doesn't only hold sometimes. But answers aren't allowed to be that short. But that's the 100% answer to your question. Whatever speed you go, there is a frame that sees you at rest in which case, what velocity is there to resist? – Timaeus Mar 6 '16 at 19:48

Look inertia is present all the time. But you can feel it only when your state of rest or motion is changed because of acceleration. This doesn't mean that you have inertia only during acceleration.

Inertia is just a property due to which we remain at the same place as mentioned by Timaeus .

When you are standing on a bus and the bus accelerates then your feet accelerates with the bus due to friction but there is no force on your upper part and so it remains at its position ( due to inertia) . This is the reason why you feel inertia while accelerating.