friction 磨擦力

首先
friction係passive同埋佢係variable
如果地下ge friction係10N
係放一個盒係上便拉
IF你比GE力係1N,FRICTION就會係1N
IF你比GE力係5N,FRICTION就會係5N
IF你比GE力係10N,FRICTION就會係10N
但係如果你比GE力係20N,FRICTION都只會係10N,THEN你就可以拉陏個盒
IN THIS CASE,FRICTION MAXIMUM係10N.

如果兩樣物件用唔同物質做，但平滑程度一樣,
如果比ge力係一樣,咁佢地乘受到ge friction都會一樣。

When you push a body, before it moves, friction = force (action and reaction pair).
When force > static friction (the greatest friction when there is no relative motion), the body starts moving.
When the body is moving, the friction is dynamic friction which is smaller the static friction. The friction will remain unchange until the motion stops.

我相信平滑程度是以friction來定的，friction一樣，平滑度就一樣，反之亦然。

For the first one, friction 係唔會愈大(For F.7 level below)
because friction = coefficient of friction x normal reaction
it is independent of velocity
But over F.7 level or above
mass of a object will increase if the velocity is increase(應該係要好快, 因為相對論,如果佢唔快的話, 個increase in mass is neglected)
當個mass 愈大, normal reaction 愈大, friction 愈大

For the second one, 唔同物質個coefficient of friction 應該係唔同的, 但有可能一樣, 不過機好細, 所以個friction 應該係唔同的(有機會一樣if the coefficient of friction is the same)