A friend of mine has bad arthritis in his wrists and cannot easily throw a tennis ball for his puppy to fetch. They both enjoy the game but the current models of tennis ball throwers aren't easy on his wrist and those that are aren't very effective. My goal was to build something he could easily fire but still get appreciable distance from.
Please keep safety in mind and don't sue me. This device can be dangerous. It involves powerful elastics being used to accelerate projectiles quickly. Never point this at a living creature or valuables. Don't load this with anything that will intentionally cause damage. Don't combine any of the previous warnings to make a super destructive device. Seriously, you'll put an eye out.
On a lighter note this will exercise a dog because it launches tennis balls approximately 100ft as shown in the video.