In the real world, a force acting on a body is usually applied over a region, such as a part of the surface of the body. When someone is pushing a trolley by hand, there is a region of the handle of the trolley where the hand pushes. When a tugboat is towing a ship, the rope that is being used is attached to a bollard on the deck of the ship. Some forces, such as gravitational forces, act throughout a body. In contrast, in a mathematical model, it is assumed that every force acts at a particular point, the point of application of the force. In some mathematical models, each point of the surface of a body may experience a force (when the surface is experiencing a pressure), or every point throughout the volume occupied by a body may experience a force (when the body is experiencing a field of force).