The air cannon demonstrates projectile motion.
Teachable Topics:
- Air Pressure
- Projectile Motion
- Safety!
Theory:
We live under a lot of pressure. Not just emotionally, but also from the large mass of gasses that make up Earth's atmosphere. A simple way to demonstrate the pressure of these gasses is to use them to do some type of work, like launching a projectile.
Picture a pipe with the ends sealed with tape, and with a ping-pong ball on the inside. Air on the inside of the pipe is pushing against the inside surface of the pipe, while the air outside the pipe pushes on the outside surface. Normally, the push of the air on the inside and the push of the air on the outside are in balance. That is, the pressure on the inside and on the outside the pipe is the same. Because the pressures are in balance, we don't notice anything peculiar.
Now use a tube and a vacuum pump to remove all the air from the inside of the pipe. Now air is pushing on the outside, but no air is pushing on the inside to create balance. You will see the tape begin to buckle in under the pressure of the outside air, as in this figure.
What we have here is an air-cannon! Breaking the tape seal at one end of the pipe will allow air to rush inside. This in-rushing air will be moving at high velocity, and will accelerate the ball through the tube. The ball and air will break through the other end of the tube, and the ball will fly like a cannon ball.
Apparatus:
- Air cannon
- Ping Pong ball
- Packing tape
- Needle
- Vacuum Pump
Procedure:
Note: This demo should be done in an open area. ALL SAFETY PRECAUTIONS SHOULD BE TAKEN BEFORE UNDERTAKING THIS DEMO.
- Seal the back end of the cannon with packing tape.
- Insert the ping-pong ball into the cannon.
- Cover the front opening of the cannon with packing tape, ensuring that the tape forms an airtight seal.
- Using a high-vacuum pump, suck all the air out of the cannon through the rubber tube.
- Carefully elevate the muzzle of the cannon.
- Pop the bottom end and watch the ball burst out, flying into the air.
- Make sure not to point the cannon at anything breakable, like cars or people. Hopefully you will be successful in finding the ping-pong ball afterwards. We recommend beginning your search in surrounding bushes!
