Forces are broadly classified into two categories: contact forces, which are required to make contact with the object in order for them to work, and action-at-a-distance forces, which can act on objects placed at a distance. Contact forces include applied force, frictional force, normal force, spring force, tension force and air resistance. Action-at-a-distance forces include gravitational force, magnetic force and electrical force.
An applied force is the force exerted on an object with the intent to move it. Frictional force is the force that opposes motion. Static friction acts on objects at rest and prevents them from moving. Kinetic friction acts on objects in motion and works in a direction opposite to the motion. Air resistance is a kind of friction exerted by air on moving objects.
Normal force is exerted on an object by the surface it is resting on to support its weight. Spring force is exerted by either a compressed or stretched spring. Tension force is the force acting on a rope or a string that is pulled tight by a load attached to it. Gravitational force is the attractive force that acts between two objects. It is directly proportional to the mass of the object, which means that heavier objects have a more pronounced gravitational force. Magnetic force is the force between two magnetic poles, which may be attractive for unlike poles or repulsive for like poles. Electrical force is the force between two electrical charges, which also may be attractive for unlike charges and repulsive for like charges.