A force is defined in Newtonian physics as any action or phenomenon that causes an object to accelerate (change in speed and/or direction). Force is related to the principles of inertia and momentum, and is found throughout the World of Goo. Gravity is an example of a force, causing structures to lean or collapse. Force is also used in Goo throwing and deploying Launch Goo and Infected Goo. In the latter two, a vector indicates the magnitude and direction of the force applied.
Gravity, buoyancy, and airflow are environmental forces - that is, they are caused by the level, not the player. For example, when a new goo ball is attached to a structure, this new weight will cause gravity to pull on the structure in a different way, resulting in changes in the speed of the structure's movement, the manner (or direction) in which it moves,and give it mild momentum. If the momentum is too great, the structure may sway to one side and fall over. Likewise, buoyancy will give goo upward momentum and direction, and airflow will change the speed, momentum, and direction depending on which way it blows.
A careful player should take these forces into consideration when designing goo-structures. For example, there is a sticky, "gooey" force between some goo balls and the ground that causes a mutual attraction between the two. This gooey force does not exist between many real objects, so it may be counter-intuitive at first, but it can be highly useful, such as when attempting to counteract gravity.
When using the Goo throwing technique, the player is applying force to the goo. This force affects the momentum, speed, and direction when thrown. When placing Block Goo, force is used as well, as placing Block Goo is more or less a tame form of Goo Throwing. Launch and Infected Goo are launched using the player's force, and arrows indicate the momentum, speed, and direction of the launch.