physics (movement and position)

friction compensated = 冇 friction 呢個net force.

因為如果你係平面推架車仔, 佢會有個friction 阻住你, 令你冇咁易推(net force 小左). or 你推完個friction 會整停佢.

但個斜路 佢係有個force 向下, 係mgsinx 咁當佢呢個mgsinx 同個木版gei friction 一樣, 咁佢就冇net force , 咁就好似冇左 令佢減速gei 力. 咁佢就會不斷咁以constant speed 咁落.

如果mgsinx > friction, 咁佢就有個net force 向下, 車仔向下加速
mgsinx< friction, 車仔減速, 最後停左