Which force acts toward the center in circular motion?

• A. Toward the axis of rotation
• B. Away from the center
• C. Toward the center
• D. Perpendicular to velocity

Answer: (C)

The key idea is that circular motion is kept by a force that points toward the center of the circle. This inward force is called the centripetal force, and it produces the centripetal acceleration toward the center. Its magnitude is F_in = m v^2 / r, so the faster you go or the tighter the circle, the stronger the pull toward the center must be. The exact cause of that inward pull depends on the situation: tension in a string, a component of gravity, the normal force, or friction can all provide the centripetal force. The important part for this question is the direction—the force must point toward the center. (Not toward the axis, not away from the center, and while it lies perpendicular to the velocity in circular motion, that perpendicularity describes its orientation relative to motion rather than the destination toward the center.)