AK-47 Underwater at 27,450 frames per second (Part 2) - Smarter Every Day 97

Episode Overview

• The host, Destin, builds on his previous experiment of firing a gun underwater to now focus on the physics of the bullet exiting the barrel.
• He constructs a special underwater viewing box (a "periscope") to film the event with a high-speed Phantom camera.
• The experiment involves collaboration with The Slow Mo Guys, who help operate the advanced camera equipment.
• The video captures incredibly detailed slow-motion footage of an AK-47 being fired underwater, revealing complex bubble dynamics.
• Destin explains the scientific principles behind the observed phenomena, including cavitation, bubble oscillation, and the Bernoulli principle.

Key Concepts

The main ideas discussed in the episode revolve around the physics of firing a weapon underwater. Key concepts include:

• Experimental Setup: To capture the bullet's exit, the host creates an inverse aquarium—a dry box submerged in a pool with mirrors acting as a periscope. This allows a high-speed camera to film the underwater event from a dry, safe location.
• Bubble Oscillation: The high-pressure gas bubble created by the gunshot rapidly expands and then collapses due to the surrounding water pressure. This cycle of expansion and collapse (oscillation) repeats multiple times, driven by the interplay of internal gas pressure, external water pressure, and fluid momentum. This is explained using the Rayleigh-Plesset equation.
• Cavitation: The video reveals two distinct types of bubbles. The first, a cloudy bubble, is formed by the hot gunpowder gases. The second, a clear bubble, is created in front of the bullet. This is due to cavitation, where the high velocity of the water being pushed ahead of the bullet causes a drastic drop in pressure (Bernoulli's principle), leading the water to momentarily turn into water vapor.
• Gas Piston System: The host explains how the AK-47's gas-operated system works, showing that a small amount of gas is vented from a port near the barrel after the piston moves a short distance. This venting is visible as a small, secondary bubble in the underwater footage.

Quotes

• At 00:18 - "You see, instead of building an aquarium that was keeping the water in with the gun, I built this to keep the water out." - The host explains his innovative solution for filming the bullet's exit underwater, using a dry box with mirrors as a periscope.
• At 01:09 - "Running the Phantom V1610 today which goes up to 18,000 frames a second at 720p. I can't count that high." - Gavin from The Slow Mo Guys describes the powerful high-speed camera being used for the experiment, highlighting its impressive capabilities.
• At 06:10 - "There's an equation called the Rayleigh-Plesset equation that describes everything a bubble does underwater...this is basically what's going on." - The host introduces the complex physics equation that governs the oscillation and collapse of the gas bubble created by the gunshot.

Takeaways

• Firing a gun underwater creates a complex series of events, including the formation of multiple distinct bubble types.
• The high-pressure gas from the gunshot causes a bubble that oscillates (expands and collapses) multiple times before dissipating.
• A separate, clear bubble of water vapor forms in front of the bullet due to a rapid pressure drop, a phenomenon known as cavitation.
• Even a small, brief pulse of gas is sufficient to cycle the action of an AK-47 due to the principles of inertia.
• High-speed cameras can reveal intricate physical phenomena, like fluid dynamics and shockwaves, that are invisible to the naked eye.