In the context of free-body diagrams, which statement best describes friction?

• A. A contact force that acts parallel to the surfaces in contact and opposes their relative motion or tendency to slip
• B. A contact force that acts perpendicular to the surface and prevents interpenetration (the normal force)
• C. A force that always points in the direction of an object's velocity and increases its speed
• D. A long-range force that attracts masses with magnitude $F=G\frac{m_1{m}_2}{r^2}$

Answer: (A)

Verified step by step guidance

1. Understand that friction is a force that arises when two surfaces are in contact and there is a tendency for relative motion between them.
2. Recognize that friction acts parallel to the surfaces in contact, not perpendicular, and it always opposes the direction of relative motion or the tendency to slip.
3. Distinguish friction from the normal force, which acts perpendicular to the surface and prevents objects from penetrating each other.
4. Note that friction is a contact force, meaning it requires physical contact between surfaces, unlike long-range forces such as gravity.
5. Summarize that the best description of friction is: 'A contact force that acts parallel to the surfaces in contact and opposes their relative motion or tendency to slip.'