It may be easy to see the impact of a meteor or missile striking the surface of the Earth, but it is harder to see the effects below the ground.

A group of physicists from Duke University, however, used artificial soil and sand to simulate high-speed strikes and used high-speed cameras to film the impact, making it possible to see what happens underground frame by frame or in super slow motion.

The research, which was funded by the Defense Threat Reduction Agency and published in Physical Review Letters, shows that soil and sand tend to get stronger when they are struck hard.

Researchers said that this helps explain why ground-penetrating missiles that strike the ground faster and harder are not always successful. It also means that when the speed of the projectile is high, it will encounter greater resistance.

"Imagine you're trying to push your way through a crowded room," said study researcher Abram Clark from Yale University. "If you try to run and push your way through the room faster than the people can rearrange to get out of the way, you're going to end up applying a lot of pressure and ramming into a lot of angry people."

The research is aimed at building better earth-penetrating missile designs that could destroy buried targets and underground weapons.

For the study, Clark and colleagues imitated how missiles struck soil or sand by dropping a metal projector with a rounded tip from a height of seven feet into beads that were made of clear plastic. Once compressed by the impact, these beads shifted light, transferring the kinetic energy of the projectile to the beads, which absorb the energy and strike against each other.

The researchers were able to see branching chains of light known as "force chains" in the areas of greatest stress. The force chains travel from one bead to another during the impact.

Because it is not viable to see the impact with the naked eye, each of these were recorded using a high-speed camera at 40,000 frames per second, making it possible to play back the impact in slow motion and allowing the researchers to see how the impact spread through the beads in great detail.

The researchers found that with faster impact, the force chain was more extensive, which means that a high-speed impact can cause energy to move fast away from the point of impact. The hardening of materials was also observed at very high speed as new contact formed between the beads.

Photo: Steve Jurvetson | Flickr

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