Dummy Punching Bag

Since sand and soil consist of particles of varying sizes, in the downstream section of a dam, fine-grained soil particles may be carried away by water erosion, a phenomenon known as piping or sand boils. The diameter of the seepage opening can range from a few centimeters to several meters, with a raised sand ring forming around the pore. A single small piping opening can destroy an entire dam. In movies, we can also see flood control soldiers throwing sandbags into the water to press down on the soil, preventing fine particles from being carried away by the water flow and containing the piping.

Military Defense Field

In various war-themed movies, we can also see that sandbags are arranged in rows along the front lines, as thick as walls. Soldiers use these sandbags for ambushes or combat, and the sandbags indeed block many bullets and artillery shells. This sandbag defense wall is still widely used today, with sandbags serving as defensive barriers. Bullets can easily penetrate steel plates several millimeters thick, yet they cannot penetrate sandbags.

Why is that?

This is related to the properties of sand. Sand is typically considered a non-Newtonian fluid in terms of its flow characteristics. Non-Newtonian fluids are widely present in daily life and nature, such as lymphatic fluid and cyst fluid in the human body, as well as the cytoplasm containing cellular fluid, all of which are classified as non-Newtonian fluids. The friction coefficient of such non-Newtonian fluids varies greatly, exhibiting both liquid and solid states.

In simple terms, when a bullet penetrates a sandbag, the immense impact force increases the density of the sand and causes it to aggregate, creating significant deformation to resist the bullet's penetrating force. Through friction between the bullet and the sand, the bullet's kinetic energy is rapidly converted into thermal energy within the sandbag, ultimately embedding the bullet head into the sandbag.