Physics: Force

There are an array of different forces acting on different parts of the trebuchet both directly before it fires and as it is firing.

The arm experiences both the gravitational force and an applied force. The gravitational force pulls down on the counterweight. This causes an applied force to pick up the arm on the other side of the fulcrum.

The base, itself, experiences the gravitational and the normal force. The base exerts a gravitational force on the ground, which exerts a normal force up on the base of the trebuchet.

When the trebuchet is in firing position, the sling rests on the base. This means there is a gravitational force acted on the base by the shot pocket, as well as a normal force acted on the shot pocket by the base. When the trigger mechanism is pulled, the sling slides across the base and up with the arm. This means there is an applied force in the horizontal direction, which is overcoming the frictional force between the base and the shot pocket.

Once the arm has extended all the way and the sling has opened, the balloon is launched. At this point, there is an applied force driving the balloon forward and a gravitational force pulling the balloon downwards. (As well as wind resistance acting against the balloon.)

In order to increase the distance of the balloon's launch, more weight (up to a certain amount) could be added increase the applied force launching the balloon. Moving the fulcrum also impacts distance. If one was to move the fulcrum up the arm a bit, it would shorten the amount attached to the sling that has to rotate in order to launch the balloon. This would increase the balloon's momentum upon launching, ultimately increasing the range of the balloon.