Cannon Sucks Balls at 600 mph

Every day we humans hang out and think nothing of the air that is all around us. It is easy to forget that the air has mass and is pulled down to the earth by gravity creating an ambient pressure of about 14.7 psi. This ambient pressure is the force that crushes a plastic bottle when you lower the internal pressure by sucking out the air. [Prof Stokes] from Brigham Young University has used this powerful ambient air pressure as the power source of his ping pong ball cannon.

Instead of filling a reservoir tank with compressed air and using that to fire a projectile, this canon has the air removed from the barrel to create the pressure differential that propels the ping pong ball. The ball is put in one end of a 10 ft long tube. That end of the tube is then covered by a sheet of Mylar. The other end is covered with the bottom of a disposable plastic cup. A vacuum pump is then used to remove the air inside the tube and it is this pressure differential that keeps the plastic cup secured to the end of the tube. When it’s firing time, a knife is used to cut the Mylar at the ping-pong-ball-end of the tube. Air rushes in to fill the vacuum and in doing so accelerates the ping pong ball towards the other end. There is a large jar at the business-end of the cannon that catches the ping pong ball and contains the shrapnel created during the ball’s rapid deceleration!

Since this was a science experiment at a university, some math was in order. Based on the atmospheric pressure and ball cross sectional area, the calculated speed was 570 meters/second or about 1300 mph. The calculations didn’t take into account leakage between the ball and the tube or viscosity of the air so a couple of lasers were set up at the end of the cannon to measure the actual speed – 600 mph. Not too bad for just sucking the air out of a tube!

33 thoughts on “Cannon Sucks Balls at 600 mph

    1. Also with their experiment they found out that the ball slows down significantly because of the plastic seal at the muzzle of the cannon due to the plastic not breaking off as easily with only the rushing air, and moving ball.. I’m not sure why they didn’t resort to using a “temporary seal” like a hard rubber pad that is held at the muzzle with just the force of the vacuum (in which the rubber would eventually fall off as air rushes in. I think this would eliminate the need for their “cheat” by using a pressurized tank)

          1. The old Sprint missile silos had a thin ceramic weather cover. The cover was laced with “det’ cord. The Sprint missile launched from its silo equal to the muzzle velocity of a .45 bullet. When the launch signal came, the det cord shattered the cover just before the missile nose exited the silo. A slow motion video showed the cover fragments still “hanging in the air” as the nose cone punched through.

        1. Well, one can make use of how camera shutters work, or a “pyramid” like cover that would spring open in the middle when there’s no negative pressure to hold it in place. You get the idea. Well the thing is it’s not going to be the rushing air, or the moving ball that needs to pop open the seal, but rather a mechanism that would remove the seal automatically so that deceleration happen once the ball is out of the muzzle.

      1. I wonder most of the energy is lost causing an initial defect in the intact seal, and much less is required to enlarge the defect enough for the projectile to exit. A pointy projectile would probably work much better. Even a ping pong ball with a thumbtack glued on. Or perhaps use an optical sensor to detect when the ball is about to hit the seal, then punch a hole in the mylar with a capacitive discharge. Any of these would be easier than moving a non-sacrificial seal out of the way fast enough at the last moment.

  1. Have you ever tried taking all the air out of a canon?

    Please fix the body text. I arrived too late to mock your headline mistake, but I can guess what it was and I am sure it was comedy gold.

    HaD, get a damn editor!

  2. When Mythbusters suggested air rushing into a tube to fill a vacuum could accelerate a ping pong ball to supersonic speed when 120 psi of compressed air couldn’t, I laughed at the TV. How could that be? It’s far less pressure. When they were proven right, I realized that with a vacuum there is no air drag on the ball as it accelerates down the tube. The ball doesn’t have to push all the air ahead of it out of the tube. They never really explained that part of the physics on the show.

      1. Easily. There’s no reason why air molecules couldn’t travel faster than sound.

        It’s just that pressure waves in air cannot travel faster than the speed of sound, so if the ball is travelling faster, the pressure wave from the air rushing in from the end of the tube cannot catch up, and cannot accelerate the ball any further.

        But you can get around that by using a con-di nozzle at the start of the tube, which turns the flow of air itself supersonic.

        1. OK, I get the idea if what you mean is there is a jet or blob of air that travels supersonic relative to the outside world, but in which the intra-blob speed of sound is close to the usual room temperature speed. Points to interesting possibilities for a cool blowgun.

  3. Woopde fucking doo 600 mph is about 6 times the speed of a sneeze, seriously can we not encourage mormons to do anything else. Do you know what these idiots believe?

    1. “Woopde fucking doo 600 mph is about 6 times the speed of a sneeze”
      If you have an interesting mechanism to launch ping pong balls even faster than 600 mph, I’m sure we’d all be thrilled to see your article.

  4. One thing I wonder is why put the vacuum source at the entry point?
    By putting it at the end, with a good enough reservoir, it would “suck” the air in and prevent the effect of air rushing in front of the ping pong and acting as a cushion.

    1. Unless I’m mistaken, the bottom-of-the-plastic-cup end cap on the front of the canon stays affixed until the pingpong ball rushes up to the front and knocks it out of the way. If this is true, it might remain effective enough as a seal to prevent much, or any, air entering the front of the canon.

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