Can of Soup

Some people buy canned soup and store the cans in the cupboard. Some people even turn these soup cans upside down for storage. If we open a can of soup that was stored in the upright position by removing the top, quite often all the concentrated ingredients are on the bottom and must be scooped out with a spoon. Even then, not all the concentrate is removed. Suppose, instead, we turn the same can upside down and open the bottom. Upon turning the can over, the soup simply rushes out into the pot. Why so?

Answer

Turn the can of soup upside down and open the bottom. Then turn the can over and watch the concentrate being pushed out by the weight of the more liquid stuff, assuming that the upward-acting static frictional force of the wall with the concentrate balances the weight of the concentrate. The weight of the liquid thus provides the nonzero net force downward to accelerate the liquid downward by Newton’s second law.

If there is delay in this evacuation process, allowing some air into the liquid region behind the solid concentrate might expedite the motion. Sometimes the air seal at the wall is very good, so that as the concentrate slips outward, a significant inward pressure difference can build up to slow down the extraction process. In addition, the molecules in the soup may interact more vigorously than expected via the electrical force between the concentrate and the wall of the can (i.e., the viscous force, the surface tension, etc., may be large enough to make the extraction even more challenging).