Here is an interesting article about the physics of the longbow and the famous English victory at Agincourt.

http://www.stortford-archers.org.uk/medieval.htm

The author doesn't mention that at Agincourt the English had the high ground. It had been raining so the attacking French had a tough time going uphill in mud. The English King, Henry V, was essetially a professional soldier. He got on top of a wagon so he could see the entire battlefield so he had good command and control, and could direct his troops to where they were needed. The French leader was in the forefront of his men and was crushed and suffocated as the troops at the rear of the French column kept charging after the front ranks were stopped by the pile of dead men and horses. Thus the French effectively had no command and control. The French Marshal (a professional soldier) argued against attacking the English at Agincourt because of their strong defensive position, and his awarenes of the lethality of the longbow. But political leaders over-ruled him, and relieved him of command. The outcome is usually not good when politicians to take direct charge of military operations.

Some time ago I did an analysis for the sling similar to the analysis referenced above for the longbow. My conclusion was that David could easily have killed Goliath assuming David was accurate. David was about 50 feet from Goliath when he threw a stone with his sling. Imagine a baseball batter trying to dodge a 150 mph fastball thrown from 50 feet (instead of the normal 60 feet). Goliath was 8 feet tall, and probably wasn't very agile. Goliath probably assumed David wouldn't be accurate enough to hit him in the face, where he had no armor. In war it is always good to know your enemy's capability.