點計摩擦力?

Since the cart moves with constant velocity,
force acting upward along the slope = forces acting downward parallel to the slope

i.e. 20 = 50.sin(20) + Ff
where Ff is the frictional force

hence, Ff = (20 - 50.sin(20)) N = 2.9 N

你要睇下運動方向，唔可以一概而論，如果係係一張平的枱，而塊積木係上面橫行移動，摩擦力同mg係完全無關係，但係如果佢既方向係一致，咁就唔同，而摩擦力既大小會係depent on個力的大小，而佢係會同運動方向相反，不過就會有一個limit，即如果小過個limit，F = f...
一個例子，如果想整停架車，係放break之後同架車停之間的時間：
F-f = ma
咁當然你有F, m同a當然可以找到架車受既摩擦力啦

希望答到你...