Water Rocket

There's just something about the hot weather, that makes me want to study Newton's Laws of Motion. Maybe, that's because so many of the experiments, that demonstrate them, involve water, and getting wet, like the water rocket we made, yesterday.

You've probably seen one of these, in one form or another, before. But, I thought I'd share some tips we picked up from building one ourselves

The basic idea of the rocket is really simple. It is just an empty pop bottle, with some water added, plugged with a cork, that has a hole large enough for air to be pumped in with a bike pump. As air is added to the bottle, pressure builds until finally, the water is forced out, and the bottle is propelled up - demonstrating Newton's 3rd Law of Motion:

For every action there is an equal and opposite reaction.

Moving from concept to reality though, is often where the tricky, and fun bit of an experiment comes in. Building the "rocket" part was easy, but finding a suitable launch stand , was not.

First of all, for the rocket, we used an empty two liter bottle, and a cork that was big enough to plug the neck.

Using a yarn needle, the same size as the needle of our ball pump, we poked a hole through the cork.

We added some cardboard fins to the bottle with tape, so we could stand the bottle upside down, and still have enough clearance to insert the cork, and the pump.

To begin with we used thin cardboard, but then we realized it would not be sturdy enough to support the bottle, once water was added, so we switched to a heavier cardboard, from a cardboard box.

Unfortunately, during our first few test runs, the cardboard was soaked by the escaping water, and became too soggy to stand.

We propped everything up in old flower pot, that had a piece broken out the side, where we could slip through our pump hose, It worked, but it was not ideal. Oh, and we decided launching from a hard surface was better than from the uneven grass.

We also discovered pretty quickly, the plastic shaft of our ball pump was not sturdy enough to stand up to the pressure being exerted back on it, as we filled the bottle.

Finally, we switched to a heavier duty, bike pump, and used the top half of a milk jug to create a launching platform.

You might recognize the milk jug from last summer's milk jug scoop ball craze. We modified it by cutting a notch in the base, for the pump hose, and adding some tape around the nozzle, to hold our bottle more securely.

It actually ended up holding our bottle a little too securely, so the cork popped out, but the bottle stayed put.

So, we removed the tape, which meant that only a small amount of water could be added to the bottle, without making it heavy enough to fall out of our milk jug base, unless the bottle was leaned against something else for support.

But, that was okay, because we also discovered, that less water means higher propulsion.

We estimated the one above, at about 60 feet.

I should probably mention, by way of caution, that apart from taking proper safety precautions with eye wear, and all that, the launches really should be done in a wide open area, with nothing overhead, and little wind. And, regardless of the success of the launch, the person doing the pumping is going to get soaked.

It's great to be a homeschooler.