What defines contact forces?

• A. Forces that occur only in liquids.
• B. Forces that require physical contact between objects, such as pushing or pulling.
• C. Forces that act at a distance without physical contact, such as gravity.
• D. Forces that only exist in space.

Answer: (B)

The idea being tested is how forces are categorized by whether there must be contact between objects. Contact forces exist only when two objects touch, arising from that interaction at a surface. Pushing or pulling a physical object involves contact, and forces like friction between surfaces or the normal force from a surface are classic examples. The chosen statement matches this definition exactly: forces that require physical contact between objects, such as pushing or pulling.

Non-contact forces, on the other hand, act without touching. Gravity pulls objects toward each other without requiring contact, magnets exert forces across a gap, and electric forces can act at a distance as well. The other options mischaracterize the idea: they either limit forces to liquids, refer to forces acting at a distance, or claim they exist only in space.