Forces can be classified based on origin as contact and non-contact forces. The forces that require physical contact between two objects are known as contact forces. Frictional, tensional, and normal forces are examples of contact forces. The forces that can act on an object without any physical contact between the object and the source are known as non-contact forces. Gravitational, magnetic, and electrostatic forces are examples of non-contact forces. Further, the forces can be classified as concentrated and distributed forces. A concentrated force is a force that acts at a specific point on an object, whereas a distributed force acts over the entire surface area or volume of an object. The gravitational force acting on the top surface of a body is slightly less than the force acting on the bottom of the body. But, conventionally, it is assumed that the gravitational force acts at the center of gravity of the body.