question regarding acceleration-kinematics concept

[jillzhou]

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for one of the Kaplan questions in the section tests: which of the following situations will the normal force on a box be the greatest?
the answer: when the box is placed in an elevator accelerating upwards.
I said, when the box in the elevator is accelerating downwards.
how do you know when acceleration is + or - If all free falling objects have +g acceleration downwards, why can't the elevator accelerating downwards exhibit the same idea? I think i'm overthinking this, any help appreciated!

 

[brood910]

To understand this, you must know that
Scale - weight = ma
S = W + (W/g)a = W(1 + a/g)

If you accelerate upwards, a/g will be positive, and thus, you will feel heavier.
if you accelerate downwards, a/g will be negative, and thus, you will feel lighter.

Regardless of which way the elevator is moving, if it is accelerating downwards, you will still feel lighter (Slowing down while going up, for example).

 

[sazerac]

Go extreme. If you are in the elevator and cut the cable, the elevator would have a downward acceleration of g, and you would be weightless with no normal force from the floor at all.

Doesn't sound so great now, does it?