Straw and Potato

A paper or plastic drinking straw can be pushed through an uncooked potato. Explain the physics. If you plan to try this demonstration, be sure that you take appropriate safety precautions keep your hands and body out of harm’s way.

Answer

Simply placing the straw in one hand and trying to push the straw through the hard potato leads to a frustrating failure. The straw material paper or plastic cannot take much compression before bending sideways. So this sideways bending must be prohibited if success is possible. We can use air pressure to help make the straw more rigid.

Pinch the straw between your thumb and forefinger about two inches from the end that is farthest from the potato and squeeze tightly. Hold the potato carefully but securely in the other hand placed in a horizontal plane with thumb on one side and fingers on the other side of the potato. Make sure that no part of the hand will be in the path of the straw that is, avoid having any part of your hand on the top or the bottom of the potato. With a sudden thrust, drive the pinched straw into the potato held in the other hand. The straw goes right through. Why? The trapped air upon contact is compressed inside the straw and helps the straw remain rigid. The paper or plastic is stretched taut and is more difficult to bend significantly. Alternately, one could pinch the straw in a vertical position in a vise or a clamp and drop the potato onto the straw (or a collection of straws).