Need Basic Physics Help

[DaShi]

Pervert!

[snoopy369]

That makes Dauphin's statement [that there is little kinetic friction btw the wheel and the ground] make less sense to me, though, unless he's trying to say that there is MORE friction between the wheel and the ground [as it's static], but that's obviously false, isn't it?

Please read more carefully. He said that there is little KINETIC friction. Not little friction in general.

Also, the amount of static friction is NOT simply equal to mu_static * weight; that is an UPPER BOUND; static friction increases as you increase the force trying to move the object up until you reach the breaking point of mu_static*weight

On the first part, right, I did know that [after a bit, at first I thought he meant in general, but by this point I figured that part out].
On the second part, that makes sense, ie, if friction were constant, it would be moving backwards if you didn't push on it (but backwards which direction... ), so the theoretical amount of friction is more of a potential, while actual friction only exists when a force is applied to something.

[KrazyHorse]

I think you have it now.

[FrostyBoy]

Your next question is probably going to be one about galilean relativity (this is a mildly insightful question)

Imagine you're on a train traveling at 99m/s to the right and you push a box with 1N of force at 1m/s (relative to the train) to the right. From the perspective of an observer on the train, this is the same as pushing a box at 1m/s with force 1N if the train wasn't moving at all; in other words, you have to provide 1W of power to push the box

From the perspective of an observer outside the train (standing on the ground), the situation is more complicated:

1) He sees you pushing the box with force 1N at 100m/s.
2) He sees the train pushing your feet with force 1N at 99m/s (each foot is at rest relative to the train as it pushes)

So this observer says to himself "the guy on the train is putting 100W into the box and getting back 99W from the train, so he's putting in 1W himself".

The observer guy sucks, I hate him.