Physics question (fundamentals - Homework 3)

[nubs]

Alright so I have a box that's sitting on a bench. Does the weight force need to equal the normal force?
My answer: yes
Computer's answer: no

Seeing as it doesn't say that there's another force acting in the vertical direction, why is the answer no?

[Aurelian]

Alright so I have a box that's sitting on a bench. Does the weight force need to equal the normal force?
My answer: yes
Computer's answer: no

Seeing as it doesn't say that there's another force acting in the vertical direction, why is the answer no?

I think the best way to explain this is to consider the following situation (where I will use g = 10ms^-2 for simplicity's sake). Suppose I have a 10kg object on a table. The weight of this object will be 100N. If the object is left unattended, the normal force will also be 100N. But we can quite easily conceive of a 20N upwards force being applied to the object (perhaps I'm pulling gently on a string attached to the object). The object will still be sitting on the table, and will still have a weight of 100N, but the normal force will only need to be 80N, because the *contact* force applied by the object on the table will only be 80N.

The issue becomes a bit more one of semantics now, but I guess to be exact, it shouldn't really be relevant whether the question mentions the existence of another force acting in the vertical direction; the normal force doesn't "need" to equal the weight force for an object sitting on a surface.

[Planck's constant]

Alright so I have a box that's sitting on a bench. Does the weight force need to equal the normal force?
My answer: yes
Computer's answer: no

Seeing as it doesn't say that there's another force acting in the vertical direction, why is the answer no?

There are many situations when the Normal reaction force does NOT equal the weight force. These include:

1) The box and the bench are accelerating (ie inside a lift or space station etc)
2) The bench is on an incline

Therefore, the Normal reaction force equals the weight force only in specific situations(smooth horizontal bench, not accelerating etc) and the default answer should be that the Normal Reaction force DOES NOT equal the weight force.