This high-speed video of a bullet fired into a water balloon shows how dramatically drag forces can affect an object. In general, drag is proportional to fluid density times an object’s velocity squared. This means that changes in velocity cause even larger changes in drag force. In this case, though, it’s not the bullet’s velocity that is its undoing. When the bullet penetrates the balloon, it transitions from moving through air to moving through water, which is 1000 times more dense. In an instant, the bullet’s drag increases by three orders of magnitude. The response is immediate: the bullet slows down so quickly that it lacks the energy to pierce the far side of the balloon. This is not the only neat fluid dynamics in the video, though. When the bullet enters the balloon, it drags air in its wake, creating an air-filled cavity in the balloon. The cavity seals near the entry point and quickly breaks up into smaller bubbles. Meanwhile, a unstable jet of water streams out of the balloon through the bullet hole, driven by hydrodynamic pressure and the constriction of the balloon. (Video credit: Keyence)
Tag: projectiles
Underwater Gunfire
When a projectile is fired from a gun or other firearm, it is propelled by the expansion of high-temperature, high-pressure gases resulting from the combustion of a propellant, like gunpowder, inside the weapon. The explosive expansion of these gases transfers momentum to the bullet; however, the gases will continue to expand outward from the gun even after the bullet is fired. They do so in the form of a supersonic blast wave; it’s this blast wave that’s responsible for the noise of the firearm. Firing a gun underwater is one way to see the blast wave, though it is far from the only way. In fact, a blast wave viewed underwater is not equivalent to one in air. The differences in density and compressibility between the two fluids mean that, while the general form may be similar, the specifics and the results may not be. In general, a blast wave underwater is much more damaging than one in air. (Video credit: destinsw2/Smarter Every Day; requested by nikhilism)
London 2012: Archery Physics
Archery is one of the oldest Olympic sports, but the physics involved are remarkably complex. Even looking only at the flight of the arrow, the problem is hardly simple. The heavy point of the arrow makes it front-heavy, and the fletches on the back of the arrow provide additional surface area on which air can act. This means that the center of mass of the arrow–where gravity acts–is further forward than the center of pressure–where aerodynamic forces act. This results in the aerodynamic forces helping to stabilize the flight of the arrow. To see why this is important, try throwing a dart fletching first!
When an arrow is fired from a bow, as in the high speed video above, the sudden impetus of force from the bowstring causes the arrow to flex and vibrate as it is fired. The aerodynamic forces generated by the fletches straighten the arrow’s flight, helping it reach the intended target accurately. Some fletching is designed to make the arrow spin; this can further improve accuracy but comes at the cost of speed since some of the arrow’s initial kinetic energy must be converted to rotation. For more, check out Archery Report, which features some great articles on the physics of archery and even has CFD comparing arrow tips. Mark Leach also has some great information on tuning a bow, which, if done properly, allows one to accurately shoot unfletched arrows.
FYFD is celebrating the Olympics by looking at the fluid dynamics of sports. Check out our previous posts on how the Olympic torch works and what makes a pool fast.
Seeing Shock Waves
In this still image from a video of a 2008 demonstration of a U.S. Navy railgun, the shock waves in front of the projectile are momentarily visible. When travelling faster than the speed of sound in air, information (in the form of pressure waves) is unable to travel ahead of the projectile, meaning that the air cannot deform around the object as it does at low speeds. Instead, a front known as a shock wave forms on or in front of the object, depending on its speed and shape. Across this shock wave, thermodynamic properties of the gas are discontinuous; the pressure, temperature, and density of the air rise drastically, but the air is also deformed so that it passes around the object. (See also: bullet from a gun.)
Tank Shock Waves
High-speed video of a tank firing at 18000 fps shows shock waves made visible due to light distortion. When the air density changes (due to temperature or compression), it’s index of refraction changes, causing the background to appear distorted. Most of the video shows the subsonic development of the turbulent exhaust plume. Note the speed at which the exhaust moves relative to the airborne shrapnel. (submitted by Stephan)
