Loaded Potential Energy

Hi Fulton,

I'm not a specialist on the physics side of things, but I do have some personal experiences with full scale copies of ancient war machines and building small scale replica.

Elastic potential energy (spring type) is the most common one you'll see, but in modern times it is often combined with electric energy to wind the spring mechanism and possibly for a release mechanism.

One type of old war machine would use gravitational energy. This is the counterweight machine. Some of them were called trebuchets. Obviously counterweight machines too can be coupled with electric energy for some of the work and for a release mechanism.

I don't know about the time aspect (greatest release of energy in the shortest amount of time), but both types of energy could be harnessed for your purpose. If I were to build one, I'd go for elastic, though. My hunch is that it would be easier to build small and compact this way.

Some of the "physical explanations for how old war machines work" are way off the mark. I've seen several war machines lumped in with "gravitational energy" in spite of the fact that they rely on springs to store the energy. Additionally, names for siege engines are "all over the place". What one source calls a catapult will be denied the name catapult in another, and it's like this with just about every name

It's clear to me that your greatest concern, and what ultimately decides how you build your machine probably won't be what packs the greatest punch, but how you can build your contraption so it's safe to use. Obviously you don't want to give your dog or yourself a concussion or broken bones, and most likely you'll want something which reduces the chance of the ball being launched straight up. You also probably don't want your thrower to overbalance so that it might launch a ball almost horizontally at low height.

Your solution will also have to take into account the size of your dog and the garden, and if you need the machine to be moved around.

This isn't really an obvious Khan Academy topic, so you'll probably better off going to a DIY forum : )