# Module 7 -- Contact Force and Applied Force

#### Learning Goals

After completing this module, you should be able to:

• Relate the Contact force to Normal and Frictional Forces.
• Understand the concept of an Applied Force.

The previous three modules have introduced normal forces and friction forces. Whenever two objects are in contact, this contact has the potential to produce both a normal force interaction and also a friction interaction between the objects. Further, the normal force will always be perpendicular to the interface between the objects, while the friction force will always be parallel to the interface. It is therefore entirely consistent to consider the normal force and the friction force to be vector components of the full contact force between the two objects. To find the full contact force, then, we simply construct a right triangle with the normal force and the friction force vectors as legs and find the hypotenuse. In mathematical notation, taking the vector sum of the normal force and the friction force will yield the full contact force:

$\vec{F}_{AB} = \vec{N}_{AB} + \vec{f}_{AB}$

It was our desire to reserve the notation FAB for the full contact force between objects A and B that led us to assign the normal force and the friction force special symbols.

## Applied Forces

The special case of a contact force that is the result of a person or animal initiating contact with an object is often called an applied force. In practice, applied forces are rarely broken into normal and frictional components (though it is not unreasonable to discuss such a decomposition).