Question about Planck's constant

[mathlete]

I was talking to my friend (who's a physics major) about the difference between quantum and classic mechanics. Planck's constant came up somehow and my friend said that you could use it to determine with a system can be described by classical or quantum mechanics. I thought he was crazy since it's just a constant, how can it determine anything? That's like saying the speed of light can determine how fast a system is moving... it doesn't make sense. Anyone have any comments? Is he right or making everything up?

 

[einstone]

Methinks your friend meant that quantum mechanical models are for atomic phenomena & are to be applied when the action is of the order of Planck's constant. Regards,
Einstone.

 

[Pengwuino]

Methinks your friend meant that quantum mechanical models are for atomic phenomena & are to be applied when the action is of the order of Planck's constant. Regards,
Einstone.

Even that can't explain it since Planck scale is many OOM from the atomic scale. It seems too ambiguous to determine what the OP meant...