Backyard Ballistics

When I was planning for this school year I asked the boys what they are most interested in. They both asserted that they would like to do more science, with more experiments, and more blowing things up. Not long after that I very fortuitously stumbled upon an excellent book, Backyard Ballistics, by William Gurstelle. Not only does Gurstelle explain in recipe fashion how to build such gems as "Cincinnati Fire Kites," and "Tennis Ball Mortars," the book succinctly explains the physics behind each fulfilling explosion.

As a bonus, each chapter highlights a famous scientist and his resume along with some very interesting history. This afternoon we read about Alfred Nobel, his invention of Dynamite, and how his subsequent concern regarding the danger of his invention directed him to the creation of a foundation whose goal is the advancement of peace and science, hence Nobel Prizes. My other major reason for endorsing this book is that I can confidently turn nearly 100% of this subject, save for a little planning, over to Johan.

This weekend, with big help from Johan the boys built a potato cannon. Here they are this morning; getting ready to show me what it can do. Yes, they are wearing pants and sweatshirts in early October, yes it did snow a little this weekend, and yes I am very, very sad to say farewell to the warm days of summer. I am feeling a little slighted right now, I am pretty sure we have just skipped fall. Yesterday afternoon when we checked on our garden, many of the big green pumpkin vines were withered and black.



Here is a shot of them building the contraption after soccer on Saturday.

The joints had to cure overnight before the finishing touches could be made and the project could be tested. It is really remarkable what a little aerosol hairspray as a propellant can do.

Explosions aren't really my idea of a good time... But the look on the boys faces when they came dashing in to ask me if I had heard the bang was priceless. Unfortunately I did not have my camera at that moment. And from the home school side, there is a whole lot of science wrapped up in all of this fun.

The potato cannon effectively demonstrates parts of all three of Isaac Newton's three laws of motion. In case you are curious:

1. The first law of motion tells us that after the potato is launched, it will continue to shoot forward until another force stops it. (In our case that is the large hill behind our house.)
2. The thunk of the potato against the hill is equal to the mass of the potato multiplied by the acceleration of the potato. (We're going to trust Isaac on this and save this math for six or seven more years.)
3. When the potato is fired from the cannon, an equal and opposite reaction is exerted on the support structure holding the cannon. (For us, it is Johan.)

Up next, "The Paper Match Rocket."