Hi all, I'm building a tennis ball launcher for my dogs, and was successful in building a spring loaded one with a "whacker bar" that rotated on a pin. A wiper motor turned the bar until the bar got to the half way point around the pivot, then the spring would pull the bar, hit the ball, and the switch would turn off and turn the motor off. Anyway, it works OK, but it's pretty big, and takes a couple of seconds to execute (at which time the dog anticipates the launch) so I wanted to try to make one with a linear solenoid. The problem is, I don't know what size I would need. Figure I want to toss the ball 20 or 30 feet. A tennis ball weighs approx 58 grams. I'm sure the calculation isn't going to be dead nuts accurate, but I need a starting point. Some of these things are rated in Newtons, others in Kg. For example: http://www.sourcingmap.com/12v-15a-...ush-type-solenoid-electromagnet-p-288060.html thanks for the help!