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Vacuum Bazooka

Public summary: 

Experiment with launching projectiles using the power of a vacuum cleaner.

Firing a projectile using a vacuum cleaner and a long tube.
Useful information
Kit List: 

- Vacuum cleaner
- PVC Sections
- Joining Pieces
- Small projectile, wrapped in bubble wrap.
(More projectiles can be made by screwing up paper and wrapping in duct tape, filling balloons with rice and putting in more balloons or using pipe insulation.)

Packing Away: 

Lives in a large red box labelled Vacuums along with weighing the atmosphere.

Frequency of use: 

Fire a small, light projectile up to 10m using a Vacuum cleaner and a length of tubing. To fire a projectile place the piece of card over the end closest to the T bend and add the projectile from the other end.

The projectile is accelerated along the tubing by the pressure difference generated by the Vacuum cleaner and shoots out of the end of the tubing. We need the card to create a seal at that end, the card is too big to travel towards the vacuum but the air pressure holds it in place. When we add the projectile we fully seal the tube. The increased air pressure out side the tube pushes the projectile along, accelerating it towards the vacuum cleaner. When it reaches the T junction it continues going straight on, with enough force to knock off the piece of card and fire out. It keeps on going instead of being sucked back because it's gained momentum from the acceleration, and the brief force from the vacuum once it's passed isn't enough to stop it.

Another good intro to this experiment is through what is this (point at vacuum cleaner), what is it for, how does it clean... Eventually you'll make your way onto suction and then you can get people to feel the suction force from the ends of the tube, you'll notice that it's stronger closer to the split junction. If you seal an end using a piece of card it becomes stronger at the other end. This leads on to putting projectiles in with and without the end sealed. Without the seal it won't fire as there's not enough suction, but be careful, as soon as you seal the alternate end it'll fire out.

You may want to talk about why it's not sucked back into the tube, most of the projectiles would actually fit but don't go that way. One thing you could do is extend out of the front of the T bend, this will lead to projectiles being sucked back though which is a bit annoying but may help some groups. You can also talk about turning corners, in a car (most) people slow down to turn, this is because you need to change the direction in which the particles momentum is, this requires a force which we don't have. Similarly running round a corner and why in the 400m there are wide corners, wide corners means a more gradual change in momentum.

LOTS OF PARTICLES /----------\
LOTS OF PARTICLES|------------| -------------------> Force
LOTS OF PARTICLES \----------/
_____________________________________________ \/ air escapes here \/ ________

There are more particle collisions on the left hand side of the ball than the right due to the vacuum cleaner reducing the pressure on the Right hand side.

Particle Theory
A good way to start (I think, depends on the age group) is often good to ask what the surroundings are made of, solids liquids gases etc, and introduce the idea of particles (kids, even young ones may have heard of atoms or molecules). You can talk about how particles behave in each state of matter, and then start to focus on the air around you. Many kids will know the names of the gases that make up the atmosphere, so that’s a nice question to ask, then you can explain that you get lots of these gas particles zooming around, but that you can’t normally feel them. Example, wind blowing you backwards. You can also explain that many, many particles bump into you each second, but because you're used to it you don't notice (and your body has evolved to deal with it, if they're old enough to know about evolution). Maybe make a comparison, such as a bag of flour (1kg) on an area the size of a postage stamp (1cm^2).

There are lots of things we can talk about in optimising how the bazooka fires.
Swapping to a more powerful vacuum cleaner will increase the muzzle velocity and hence the distance travelled. Why? Because this increases the pressure difference created in the tube.
What about swapping the piece of card for a better seal? Well this would be good for increasing the pressure difference but it depends on how easily the projectile can knock it off when it fires out, we lose some energy (and hence speed/distance) by knocking it off. My theory is that this is probably worse.
What about changing the angle you hold it at? If they've seen the trebuchets experiment or know some mechanics they may have some ideas about a slightly upwards angle being good for distance. However with the vacuum bazooka we have gravity also playing a roll. Firing directly downwards will obviously increase the muzzle velocity and I doubt firing directly upwards will work. You could do the maths and figure out the optimum yourself before demonstrating and write it here.
Projectile weight is another variable, this one should also interact with the above. Demonstrating outside you'll find the crosswind probably means slightly weightier projectiles get blown about less hence fly further. With less wind the lighter the better, unless you're firing downwards from a vantage point.
These should be tested on the bazooka and see what happens.

Also see the Vacuums experiment which can be paired with this one.

Risk Assessment
Date risk assesment last checked: 
Sat, 18/01/2020
Risk assesment checked by: 
Conor Cafolla
Date risk assesment double checked: 
Sun, 19/01/2020
Risk assesment double-checked by: 
Risk Assessment: 

Fire a small, light projectile using a pressure difference generated by a vacuum cleaner.

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Projectile People being hit, and potentially injured, by the projectile and/or PVC tube. Also possible damage to surroundings. Mainly persons not part of the experiment 4 3 12 Demonstrator should maintain control of the direction the bazooka is firing in.
If not holding the firing tube the demonstrator should have the projectile, end stop or both in his/her possession at all times to prevent bazooka being fired by anyone else.
Bazooka should be sited to allow it to be fired along the room into a wall or (ideally) curtain, such that people who are not involved in the experiment cannot accidentally walk through the area, and so that those that are participating can be excluded from the firing area during firing.
Projectile should be composed principally of light-weight packing material or similar (such as bubble wrap). It may be desirable to weight the front end with blu-tack or similar to encourage predictable flight but this should be done with consideration for the total resulting weight.
Call a first aider in the case of an injury.
3 2 6
Electrical parts See electrical parts RA. All 3 3 9 Follow electrical parts RA.
In the event of an accident, call a first aider.
1 3 3
Electrical cables Trip hazard. All 4 3 12 Place wires sensibly (not across middle of room). Tape down if necessary.
Call a first aider in the case of an injury.
1 3 3
This experiment contains mains electrical parts, see separate risk assessment.