What you Need
What to do
There are 2 designs for this using either cardboard tubes or waste pipe.
The Cardboard Method
Cut a short length from your tube about 10cm long.
Cut a hole (or a drill a series of holes making a sieve type arrangement) in the side of the long tube about 5cm clear of the end.
Shape the short piece of tube so it will fit nicely onto the side of the long one.
Tape them together.
Attach the vacuum hose to the short pipe (this may require more tape)
The Waste Pipe Method
Attach the length of waste pipe to the T-junction. If the junction is asymmetric attach it as shown, to reduce the chances of the projectile getting sucked down the side pipe.
Attach the vacuum cleaner to the side of the T-junction
Make a light projectile by rolling about 5g of plasticine into a sausage.
Roll the plasticine sausage in some bubble wrap until it it big enough to fit loosely in your main tube. This will be your projectile! Use some tape to hold it together.
Turn on your vacuum cleaner
Put a piece of card over the short end of the tube - the vacuum should hold it in place.
Hold the projectile in the other end of the tube.
WARNING - If you have a powerful vacuum cleaner the projectile can come out very fast, so DO NOT POINT THE BAZOOKA AT PEOPLE or other breakable objects.
What may happen
You should find that the projectile flies out of the end of the tube at high speed, in a very satisfying manner.
Why does it happen?
When you attach the vacuum cleaner to the tube it can suck up to about 10% of the air out of the tube. This may not sound like very much difference, but air pressure is huge. Air pressure pushes with a force equivalent to 10 tonnes (about the weight of a double decker bus) on every square metre.
This means that the air inside the tube is pushing out on the projectile with a force of about 160N (equivalent to 16kg), but the air outside is pushing in with a force of about 180N. This means that overall there is a force pushing it down the tube of 10-20N (equivalent to 1-2kg).
This force is very large for the 10-20g mass of the projectile, so when you let go it will accelerate down the the tube very rapidly. When it hits the card it is going fast enough to knock it off and fly through the air.
This is how we suck things up a straw, we reduce the pressure inside our mouths which means that the air pressure pushing down on the drink can push it up the straw into our mouths.