# How does the normal force change *while* I'm lifting a heavy object?

A heavy weight is resting on a surface with, with the normal force equal to the weight. So the object is stationary. (W = N)

Now I try to lift the heavy object so I'm exerting some upward force U. The object is still stationary, so it must be the case that W = N + U. And that means that the normal force has decreased.

Question: How does the normal force know how much to decrease?

Edit

Trying to clarify the question:

It's clear to me that Fnet = 0. My question is how does it stay 0? How does the normal force "know" how much it needs to decrease to keep it at 0?

• The normal force doesn't have to know anything, the fact that it decreases is just a consequence of Newton's third law on the weight of the object and the opposing force the table exerts. Sep 1 '19 at 19:53
• When you apply an upward force to something resting on a scale, how does the scale know how much to reduce its reading.? Sep 1 '19 at 21:04
• @BobD great question, how? Sep 1 '19 at 22:01

According to Newton third law action = reaction in layman's terms, so it's better to rewrite your equation other way around: $$\vec{N} = -( \vec{W} + \vec{U})$$. Btw, I have extended a little bit U notation - it can be negative or positive as well. If negative - makes normal force smaller and when positive - makes normal force bigger.