Contact force
Also called Contact forces
A contact force is one that exists only while two surfaces are touching, such as the normal force, friction and tension. AP describes contact forces as macroscopic effects of interatomic electric forces, so they are electrical underneath.
Touching is the requirement. AP Physics 1 EK 2.2.A.2 states that contact forces describe the interaction of an object or system touching another object or system and are macroscopic effects of interatomic electric forces. Push a book and what happens is that electrons in your fingertips repel electrons in the cover.
The other family needs no contact. EK 2.6.A.2 says a field models the effects of a noncontact force exerted on an object at various positions in space, which covers gravity and the electric and magnetic forces. Between them the two categories account for every arrow on an AP free body diagram.
Contact forces in the courses: the normal force, static and kinetic friction, tension, the spring force, and the buoyant and drag forces from a fluid.
Three consequences that decide questions:
- No contact, no force. A ball in flight has weight on it and nothing else. There is no lingering force of the throw, and drawing one is a standard free body diagram error.
- A surface can only push. The normal force points out of the surface, never into it, which is why an object leaves a track the moment the required force would have to go negative.
- No formula gives the size. Contact forces adjust to whatever the situation demands, up to a limit, so you get them from Newton's laws rather than from a lookup.
The routine: list everything touching the object, then add the field forces. How to draw a free body diagram works through it.