**Ask a monkey a physics question thread**

[oskar]

I think one rule of portals is pretty clear and that is that you can walk through it. If the inertia doesn't carry over, then you wouldn't be able to walk through it. As soon as any part of you would cross the threshold it would stop dead. So you'd either get turned into soup or at least get stuck. Getting stuck is not part of portal lore.

[MadMojoMonkey]

I think one rule of portals is pretty clear and that is that you can walk through it. If the inertia doesn't carry over, then you wouldn't be able to walk through it. As soon as any part of you would cross the threshold it would stop dead. So you'd either get turned into soup or at least get stuck. Getting stuck is not part of portal lore.

When you said inertia, I think you meant momentum.

Inertia is kinda a tricky word because in physics it kinda means mass. There are no "inertial forces" in physics - what people usually mean when they say that is what a physicist would call momentum.

The problem with portals and momentum is that momentum isn't a universal property of an object. An object's momentum is the product of its mass and its velocity, p = mv. (IDK why we use p for momentum, but there it is.) It's the velocity part that's tricky. Whether an object is moving or not is a matter of perspective. Whether 2 things are moving relative to one another is universal, but the rate at which they move relative to each other can be different to different observers. It will always be the same to the 2 objects, though.

So with an object moving relative to a portal, it's that relative motion of the 2 relative to each other that would seem to matter.

However, ong brought up a great point by bringing up causal links. A pair of portals has to be causally connected to qualify as portals, I think. In a sense they are the opposite sides of a 2D object that exist at different points in spacetime. Going into 1 means coming out the other. That's a causal link.

So now here's another problem with momentum.

If the 2 portals are somehow connected in a single location such as opposite sides of a 2D object, then which portal's relative motion to the object going through it matters? The one it enters or the one it exits? In the above example, these are not the same.

[OngBonga]

Momentum is inertia * velocity. If two identical objects are moving at different velocities, they have the same inertia but different momenta.

[MadMojoMonkey]

Momentum is inertia * velocity. If two identical objects are moving at different velocities, they have the same inertia but different momenta.

Each object is always at rest in a reference frame that moves and rotates along with it.

The relative motion is universal, but the absolute motion is not.


FYI, in my physics department, you'd get a -1 on anything you turned in that said "inertial force" in it. There'd be a note on the side correcting that term to whatever would be more appropriate in that context.


I appreciate that colloquial speech isn't any less correct than scientific jargon. I absolutely code switch when I'm talking at work vs. when I'm talking to a layman. If you and I are talking and you keep saying inertial force, I'm unlikely to correct you more than once. If you know that I expect a different word, but you choose to use your word, that's fine. Communication requires cooperation, and semantics isn't a hill I'd choose to die on.