[Physics] Physics/forces involved when separating two 5 gallon buckets


So a friend jokingly said "there's no force in the universe stronger than two 5 gallon buckets that are stick together." And I thought that this has to have a wonderfully complex and scientific explanation. I would guess that it had to do with the vacuum created when pulling the two buckets apart, combined with the fact that the walls are so steep that the tiny crack created between the buckets is not as large as the air that is trying to enter the space.

But that's my non-scientific speculation. Does anyone out there know a physics teacher who uses this as an illustration? Or does anyone already know how to quantify the forces involved?? Many thanks in advance!

Best Answer

There are two effects at work there, as follows.

First, buckets like that are injection-molded, which means they are formed with a slight "draft angle" or taper to their sides, which makes it easier to get them out of the mold in which they were formed. The wall thickness is typically constant throughout the bucket, which means the outside taper angle matches exactly the inside taper angle. What this means is that if you scrunch one bucket down inside another bucket, they fit together extremely snugly from a geometry standpoint, and the slight elasticity of the bucket material causes the inside bucket to contract slightly and the outside bucket to stretch slightly. All these effects create a bucket pair which, when firmly scrunched together, tend to cling extremely tightly to one another via mutual friction.

The second effect is that to pull them apart, air needs to find its way into the gap between the two buckets. Because of how perfectly they fit together, there is essentially no gap between them and hence almost no way to get any air to flow down into the space between them.

These two effects guarantee that it is almost impossible to pull a pair of identical plastic buckets apart once they have been pressed together.

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