[Coda]

Originally Posted by Potironette

That way of writing down vectors seems really useful :o. It's like the vector don't need to be drawn anymore and their actual location doesn't really matter anymore..?

Well, you can't even DRAW vectors of more than two dimensions. :P

Sometimes you'll see it without commas, just spaces, and technically you're supposed to write them vertically:

There was a multiple choice question on my test saying something like: "Why don't we account for the internal forces of an object when calculating its momentum?" And then there were four things to select. I guessed a choice that said internal forces were somehow "conserved" too, but frankly I have no idea why '~'

Newton's first law. An object in motion stays in motion unless an OUTSIDE force acts upon it. Internal forces can only make the parts move relative to each other, but that can't change the center of mass of the system.

This could also be argued using Newton's third law. Any internal force must necessarily have an equal and opposite internal force, so outside the system there's no net force.

EDIT: However, the question gets MUCH more interesting if you want to take relativity into account. Since momentum is proportional to energy, you might want to ask if the energy from internal forces can influence momentum.

The answer to that is: We don't know! So far we haven't seen any measurable violations of it. But there are theories that suggest that there might be an exotic force we haven't discovered that could increase an object's relativistic momentum without an outside force acting on it. Such a force could possibly be an explanation for dark matter (not actually matter, but this exotic force creating momentum that can't be explained by mass in motion) if it existed, but its influence would be so subtle at human scales that we'd have a very difficult time measuring it -- it's already stupidly hard to measure GRAVITY at human scales.