# Why do objects have inertia?

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Is it to do with the atomic structure or something similar, or am I completely along the wrong lines?

In: 4

Inertia is related to the notion of causality, not the makeup of an object (a non-quantum object to be precise). In order for something to happen to an object, something has to cause it to happen. If there is no cause, there is no effect. So an object at rest will continue to stay at rest unless and until something causes it to no longer be at rest, and an object in motion along a particular trajectory will remain in motion along that particular trajectory unless and until something causes it to no longer remain in motion along that particular trajectory.

Think of a bowling ball – if it’s just sitting in the rack, it will continue to just sit there (at rest relative to the Earth) unless and until a bowler picks it up and starts moving it. The bowler applies force to change the speed and direction of travel of the bowling ball, but that was not going to happen unless and until that force was applied. Has nothing to do with what the bowling ball is made of or the properties of the atoms that make up the ball (except that the atoms all stay together). Now if the ball is thrown down the lane, it will continue to move down the lane unless and until a force acts to stop its movement (friction from the air and floor initially, contact with the pins and finally gravity pulling it off the lane at the end (leaving aside the issue of gravity not being a force for the sake of simplicity).

Ok so lets start with motion itself. A point like object can move in very different ways. It can move at a steady speed in a straight line it can accelerate and move in a circle. So far every object behaves the same way.

What happens when we introduce another object and they interact. We define a constant that codes a property of an object, how it behaves during interactions. We use the letter m to mark this amount and call it mass.

Mass determines how much effort you have to put in to chage the object’s motion. F/m = a. If you apply some force the mass will determine how much the object will accelerate. Everything with mass resists changes in motion. Well its more like in order to do something you have to apply a force which means you have to do work. If energy is conserved things can only change their motion if something makes them. With more massive objects you have to put in more work to move them.

An other way of looking at it is through Newton’s third law. You kick a ball and the ball kicks you. Why did the ball move while you didnt? Because the same amount of force changes the ball’s motion a lot more than yours.

Asking why does inertia exists is like asking why mechanical energy is conserved. These are useful concepts and they all based on experiments. Inertia defines how an object behaves during interactions.

ELI5? : Inertia is a fundamental property of any mass. It has nothing to do with atomic structure. We observe all masses to have a property to resist any change in its state of motion.

It can be argued that inertia is an outcome from the observation that laws of physics are independent of location and movement across space and time (same laws everywhere in the universe, at all points of time). But honestly, that’s less of a causal “reason” for inertia. Instead I would frame it as another observation which fits.

Beyond the ELI5: according to general relativity, inertia, like gravity, has something to do with the curvature of space. But unlike gravity, the explanations are not very intuitive (in my opinion), and still subject to debate (in the opinion of more educated physicists).

Would love to stand corrected, in case some physicist can actually ELI5 this.

You can see a discussion on it here:
https://physics.stackexchange.com/questions/69479/why-does-inertia-happen

And another slightly deeper article which I can’t say I fully understand, talking about the issues in explaining inertia: https://www.americanscientist.org/article/the-forgotten-mystery-of-inertia

If there is energy in motion, to move any object that energy has to be transferred to that object, meaning work has do be done / a force has to act on that object for a certain amount time/length. Conservation of energy dictates that no object can just suddenly have more energy without it being transferred from somewhere else, so something has to act on that object to make it move (=it has to have a certain inertia), and as the energy in motion is proportional to the objects mass, the change in motion is inversely proportional to that mass for a given force (= a more massive object ‘resists’ change in motion more/has more inertia).

Inertia is not a fundamental property of objects per se. It’s more a higher level phenomenon we recognize as a result of cause and effect.

For something to happen, something must cause it to happen. That is the basic premise. I.E. push a sofa to make it move.

In principle things are pretty straight forward: push something with mass for 10 energy points and it will move for 10 energy points.

The trick with inertia is that the mass in an object, like your sofa, is locked together (that’s why it’s a sofa and not part of your floor). So unless you break it’s structure all parts of the sofa needs to move together. That means the 10 energy points you pushed the sofa for must be distributed among all the mass in the entire sofa – giving less energy points for each individual part of the sofa to move.

A heavier sofa has more mass, and therefore more parts that require energy points in order for all of them to move together, meaning a heavier sofa will ultimately move less for the same amount of energy point pushed into it.