Which of the following is an example of a contact force?

• A. Gravity
• B. Magnetic attraction
• C. Electrostatic force
• D. Friction

Answer: (D)

The main idea is the difference between contact forces and non-contact forces. A contact force only exists when two objects touch. Friction fits this perfectly because it arises from the interaction between surfaces in contact and acts to oppose their motion relative to each other. You only feel friction when there’s contact between surfaces, like a block sliding on a table.

Gravity, magnetic attraction, and electrostatic forces don’t require contact—they act across distances through fields or distances between objects. Gravity pulls objects toward each other through space, magnetic attraction acts through a magnetic field between magnets or materials, and electrostatic forces act between charges even when they’re not touching. So friction is the correct example of a contact force.