So I'm trying to understand Newton's laws, and I know that friction is supposed to be the force that stops an object in movement. But I don't really understand why this is the case, as the force of friction is not something that can be perceived intuitively (unlike, for example, the force I could apply to a box in order to move it around). In fact, if I remember correctly, it used to be believed (before Newton's time) that objects needed a constant force to move. There was no such thing as "friction", things were supposed to naturally come to a halt after being applied a force. Why did this notion change? Basically, what I'm trying to ask is, how did Newton come up with this idea? Because it's not an empirical finding: after all, he was the one to define what a force was in the first place. Or is it considered just to make inertia and the first law valid? And if so, how do we even know inertia is truly a real property of objects? My apologies if I haven't expressed myself well or posted in the wrong section of the forums (wasn't sure where I should write this, because it isn't a homework exercise or anything of the sort).