f.2 friction 急

就好似我地推1張枱, 就咁平推佢有4隻腳就會難推D,但係令到張枱得2隻腳掂地就會唔洗用咁大力
It because when you raise the table a little bit in so that two of the four legs are not in physical contact with the ground, there is a vertical force component from your pulling force that will reduce the "normal reaction" acting on the table.
Since frictional force is proportional to the normal reaction, a reduction of normal reaction thus leads to a decrease of frictional force.
但係我地D波鞋呢?
鞋底有坑紋, 係ROUGH D, 所以FRICTION INCREASE,
不過同埋有坑紋既鞋亦比無坑紋既鞋既surface area細wor,
咁照計就應該decrease friction而唔係increase

No ground is perfectly flat (if it were a perfectly flat ground, it would have a perfectly smooth surface), there are tiny "ups and downs" on the ground even if it "looks smooth". When you are walking/running with your sport shoes on the ground, the threads at the bottom of you sport shoes are actually "locking" onto the tiny ups-and-downs on the ground surface, such "locking" effectively avoid any slipping. You therefore feel that friction is increased.

Why does the change in the surface area have to be the result of a change in the way that you push the table?

What if now there are two objects with identical mass but different surface areas in contact with the floor?

無坑紋就會變左smooth surface
錦樣同加左lubricant差5多-.-