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Water rockets

Introduction
Public summary: 

Try not to get wet as you fire water-powered rockets into the air. Who can shoot it the highest?

Launching lemonade bottle water rockets with a foot pump.
Useful information
Kit List: 

- Pump.
- Bung with hole or with valve.
- Rope
- Launching base.
- A selection of lemonade (or other fizzy drink) bottles
- Lots of water in a large container.
- Preferably another bucket for filling the container.

Packing Away: 

The key is that the launcher goes in on its side. After removing one of the pump's legs (using the screw at the top - make sure you replace the screw afterwards so it doesn't get lost!) that will fit on top of everything else.

Frequency of use: 
3
Explanation
Explanation: 

Setup

Set up the rope running from the launch area to a higher point (raising the launch area, e.g. blocks, can keep the rope above head height) - run the rope through the tube attached to the bottle. Jam the bung in the bottle and get kids to pump until the rocket launches up the rope. Hopefully it should come back down to somewhere you can grab it. Keep people out of the area where the rope is so that they don't get hit by the rocket or walk into the rope!

As a further point - it can very occasionally be done without the guide rope pointing vertically upwards, or without a guide rope at all. This is suitable in large sports halls or outside only. Here care needs to be taken that a large enough area is cordoned off so that it does not hit anyone when it returns to Earth. If a guide rope is not being used, it can still be helpful to attach a rope to the rocket, in order to limit its range (such as to avoid hitting people) or to aid retrieval.

Explanation

Water rockets are great fun. Everyone wants a go, from 3 to 13 - and the teenagers and adults too, if they are brave enough to admit it. The does mean that you'll always have a queue and so an interactive discussion is difficult to achieve: on the upside, they'll be back for another try. I always try to remember the faces to keep a line of questioning going from launch to launch. I question them before every launch on a couple of points and while questioning I always keep their rocket in my hand, it keeps their attention and stops a rocket from flying up your ass.

As a first question I always ask: "So how do you think it works?" Which is always a good starter and that gives you your level. If it's not a terribly bad explanation, generally I don't try and correct it or improve it and I let them launch their rockets. Each time they come back a couple more questions maybe a little demo. They very rarely listen to the children ahead of them, which is kind of stupid, but there you go.

Then I suppose I start to refine their understanding of how it works and try and get them to experiment. Trying the bottle, full, empty; the bung in hard, or soft - you have to fiddle this one as it is difficult to control. Indeed you have to fiddle the empty bottle too, you want it to go about 5 feet and catch it, otherwise it can go almost as high as with water.

You've got a couple of lines of questioning. Pressure is always a good one. You can relate it to the pump. As a kid I could never pump up my bicycle tyres, because I didn't push the plunger very far. That sort of inspires me down this line. You can put your thumb over the valve and get them to push down on the plunger, the further they push down the harder it gets. When they let go the air may push it up a little. So that's your basis for explaining what pressure is, how you raise the pressure in the bottle and how it pushes the bung out. Don't use density.

Older kids and adults may talk about molecules. When your talking about molecules you can raise the concept of molecules inside the bottle hitting the inside of the bottle. I quote how fast an air molecule moves, about 500 ms-1, so even though it's light it packs a punch. I crush the bottle, put the bung in and then pump some air in. The bottle comes back to its original state, so the air molecules hitting the wall have pushed it back into shape. Then invite an assistant to squeeze the bottle, it's soft, but as you pump more air in it gets harder and harder, because those air molecules are pushing back at you. It's not a way people generally see the world so it's kind of a fun thing to do.

As part of fully explaining how it works you need the water there to push the rocket up. Every reaction has an opposite but equal reaction. I put the question: "when you lean on a wall and push on it, does it push you back?" All ages generally say no. So on to the demo, lean forward and get an assistant to push on your hand as your wall; "Now, I'm pushing against him and he's pushing against me and I'm going nowhere, what do you suppose happens if he's stops pushing against me." Your assistant stops pushing and you pretend to fly forward. "So if you're pushing against a wall, it must be pushing back."

The more water you've got in the bottle the harder the air pushes it down and the bottle up. So the more water the higher it goes. Sometimes getting them to imagine throwing a tennis ball versus a boulder can help.

Next is the energy. Where is the energy stored? This can lead on nicely to all sorts of questions about conservation of energy and what forms of energy are being converted from lunch to launch. So the more air the higher it goes.

There seems to be a catch 22 situation. Of course it means there is an optimum and this is an important scientific concept in itself. Remember the heavier it is the lower it goes as well. This is all worth expanding on, especially with adults and at this point I may sweeten the pill by saying that on tour "we" settled down to try and work out what the optimum amount of water is, four sides of maths later we still hadn't solved it. The mathematics of how these bottles launch is more complicated than NASA rockets as the thrust changes over time.

The next bit is the bung. This is what really makes it go high or low. "If I push in the bung really hard will it go higher, lower, or not make a difference?"

I even ask the tots questions. I push some air into the bottle and say, "What are these?" Bubbles. "What's in a bubble?" Air, relate to blowing a balloon, what is in a blow? Indeed what's air?

These are just some of the themes I question on and there are plenty more so you shouldn't get bored. Remember, be a bastard hold that bottle 'til you've got some science into, or out of, them.

Practicalities

Let them fill their bottles some way away, though you may want another bucket to make your own adjustments to the amount in the bottle. The area around the launch site gets very wet, so move on occasionally. Stay away from roads, people and roofs. Keep the queue back and make sure the rocket isn't angled towards their, or your, face. Bottles may get stuck in trees, that's half the fun.

If you're launching along a rope, you can engineer your setup so that most of the water goes back into the bucket on launch. This will save you some walking!

The more taut the rope is, the less friction the rocket will experience and the further it will go, so tie it to something strong and tie it tight!

Risk Assessment
Date risk assesment last checked: 
Fri, 12/01/2018
Risk assesment checked by: 
Josh Garfinkel
Date risk assesment double checked: 
Sun, 04/02/2018
Risk assesment double-checked by: 
bakrill
Risk Assessment: 
DESCRIPTION Build water rockets out of lemonade bottles by pumping air into bottle which is partially full of water.
RISKS
  1. Slip hazard
  2. Bottle exploding (slim risk)
  3. Hitting people with the bottle
  4. Walking into or tripping on the rope
  5. Slipping on water from rocket
ACTION TO BE TAKEN TO MINIMISE RISKS
  1. Do the experiment outside on a surface that can take water. If done indoors or on a surface which can get slippery, keep a mop close to hand and cordon off the area.
  2. Check bottle for cracks and other damage before use. Any damaged bottles should be cut or marked to show they cannot be used if they cannot be disposed of immediately. Only use 2L "fizzy drink" bottles (coke, fanta, etc) not bottles for still drinks which are not (always) suitable for pressurising.
  3. Keep the range clear, and cordoned off if necessary, and make sure the launching platform will not fall over. Ensure that the person pumping the water rocket does not lean over it (try to keep the pump as far away from the rocket as the hose will allow - we have lengthened the hose to make this job easier). Locate the experiment sensibly - bear in mind roads etc and wind direction. The rope can be used to limit the range of the rocket, even if a guide rope is not used.
  4. As with (3), ensure visitors are kept away from the rope. Any trailing rope ends should be tucked away and kept off the ground if possible.
  5. Depending on surface, keep mop on hand and prevent running about
ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT

Call first aider in event of injury

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