action and reaction 作用力和反作用力

Remember that Action and Reaction act on different bodies.

The R in your expression is NOT the normal reaction of the weight, mg. It is the normal reaction given by the surface (e.g. floor) on which the object is placed

The normal reaction of the weight (mg), in fact, acts on the earth.

Thus mg and R are NOT a pair of action and reaction. This is why they may or may not be equal just depending on situations (like the surface where the object is placed is stationary, accelerating or decelerating).

我看不見矛盾。
你問的應該是lift的問題。
你個F=mg-R，你施於地下的力=地下施於你的力，沒有衝突。
你不明白的是R的問題，R其實是法向反作用力（Normal Reaction）
一般R=mg（g=10，只受地球吸力影響），但其實R=ma+mg才是正確的，a=0就是感覺如平常，a就是你lift的加速度。請留意方向，設R為正時，如果lift的加速方向是和你的R一樣時，你就會覺得自己重了，action and reaction pair也。詳情請請教老師和找教課書。