Could one of you mechanical geniuses please help out a dumb ol' elec engr? I've searched the boards for a few hrs now, but can't seem to get confidence in my answer to what should be a simple problem. I'm trying to build a simple a simple tennis ball launcher to fire a ball out at 70mph -- assumptions below. I've found v= sqrt(k/m)*l as the equation I apparently need to use. So, I can get a "number", but I am not clear on how the units all work out as I am not seeing a time unit on the right side of the equation. I would REALLY appreciate someone knocking this out for me so that I can at least get in the right hemisphere for starting to experiment with diff spring sizes. So, here are the relevant parameters & constraints: 1. Launch mechanism is a simple, linear compression spring loaded, then abruptly released to launch a tennis ball out of the barrel of the apparatus 2. Tennis ball mass = 5 oz. 3. Desired velocity of tennis ball upon separation from fully recovered spring is 70 mph 4. Spring to be compressed exactly 5.0 inches 5. Assume no frictional or angular losses 6. Assume no elasticity/rebound effect from tennis ball construction. Question: What spring constant, K, is required to accomplish the 70 mph launch?