Trebuchet at Urquhart Castle

Here is the reconstructed trebuchet at Urquhart Castle on Loch Ness.

 Seen in the photograph below, it is easier to understand.

 The arm is settled so that the long bit is close to the ground, and it has a heavy ball like the one below in a sling attached to the arm. Then a much heavier load is put in the wooden cradle on the right. Since there is no balance, the wooden cradle will fall, rotating the arm. Both sides of the arm rotate with the same angular velocity, but it is the linear velocity of the heavy ball that will be most important. Linear velocity = angular velocity x radial distance from the pivot. Since the ball is about five times further than the cradle, it will have a high linear velocity when it is fired. It is a bit more complex than this because the ball is attached by a string and sling, so there are whiplash effects.