An elevator is at rest with a bag hanging from a spring scale. My question is Which force gives the scale reading? The mg (the weight of the bag) downward or the force exerted by the scale on the bag Fn gives the scale reading?
thanks.

Since there is no motion the net force is zero. The system is in equilibrium with all the action-reaction forces balanced. There were forces in play when the bag was first hung on the scale and it moved into position, but they're gone now. The bag hanging from the scale is now no different than if it were being supported on a floor. The scale simply has an "indicator" that describes how much force is being held in balance. A floor scale would do the same.

By the way, this is the third query you have posed. The other two were answered without the slightest hint you ever checked back.

No. It's my second I think. I indeed checked back, but I just didn't have any problem, so yeah. I will reply back. thanks. but on my book, it says "The scale reading indicates the downward force exerted on it by the bag. Two forces act on the bag: the downward gravitational force mg and the upwar force exerted by the scale equal to w" So which force gives the scale reading? w or mg? I mean when it is at rest, w=mg, but when it's moving or accelerating, w is not equal to mg, so w indeed gives the scale reading?

At any point in the (stationary) system, there will be two equal and opposite forces, both equal to mg. If you look at the point where the bag is held to the scale, the bag exerts a force of mg on the hook and the hook exerts a force of mg on the bag.

What are you referring to as w? When the elevator is accelerating (up), there is a resultant force upwards which will be equal to (ma). The reading on the scale will be (ma + mg), so mg is certainly still part of the scale reading. This is equal to (m(g+a)) which tells you that F=ma still holds and the new a is the sum of the gravitational potential and the physical acceleration. (If the elevator is moving but not accelerating there will be no resultant force.)

I think an accurate answer (if there is one) will depend on the internal workings of the scale. The two forces in question are equal and opposite and it's difficult to tell whether the scale is measuring the force downwards or the associated force upwards.

Hope that helps.

NoCleverName completely answered you question. Read it over agin until you understand it.

Both forces exist in equilibrium. The scale reads according to the force it must exert to make the system motionless.