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Lowell,
I think you've been cheating! It sounds rather like you are dragging a
'train' of two or more blocks around, each independently resting on the lab
table surface.
This is inappropriate if true.
The general idea is that a bearing surface of widely varying area only
bears at 3 points.
Two independent blocks obviously bear at six points.
This would evidently affect the unstick force.
Other factors that might vary the three-point bearing idealization are a
coating of wax or very carefully smoothed bearing surfaces.