Physics 9.1 - 9.2 - Interactions and vectors

Forces come from interactions

There are two broad ways for objects to interact.

A normal contact force acts perpendicular to a contact surface. Here, normal means perpendicular, not "usual". Friction acts parallel to surfaces and opposes their relative sliding, or the tendency for them to slide.

Every interaction involves a pair of forces. If object A pushes or pulls object B, object B simultaneously pushes or pulls object A with an equal-magnitude force in the opposite direction. The two forces are the same type, but they act on different objects.

For example, a book and table have a contact interaction:

• the table pushes upwards on the book;
• the book pushes downwards on the table.

Earth and the book also have a gravitational interaction:

• Earth pulls downwards on the book;
• the book pulls upwards on Earth.

Each force can be represented by a vector arrow. The arrow points in the force's direction, and its length represents the force's magnitude. Keep the object receiving the force clear: the two arrows in an interaction pair belong to two different objects, so they must not both be counted as forces on one object.

A common trap is to assume that the normal contact force on an object always equals its weight. It does in some simple situations, but not by definition. The normal contact force is determined by the contact interaction and always acts perpendicular to the surface.