In a simple pendulum, which force primarily acts as the restoring force pulling the bob toward the lowest point?

In a simple pendulum, which force primarily acts as the restoring force pulling the bob toward the lowest point?

• A. Tension in the cord
• B. Gravity
• C. Centripetal force
• D. Friction

Answer: (B)

The restoring force comes from gravity acting along the path of the bob back toward the lowest point. When the bob is displaced by an angle theta, its weight mg has a tangential component mg sin theta that pulls the bob back down the arc. This tangential component is what drives the back-and-forth motion toward equilibrium. The tension in the cord points along the string toward the pivot and is purely radial. It doesn’t have a tangential component to pull the bob toward the bottom, so it doesn’t act as a restoring force. It mainly provides the inward (centripetal) force needed to keep the bob moving along its circular path. Friction is neglected in the ideal model, so it doesn’t contribute to restoring either. Thus gravity’s tangential component is the primary restoring force. For small angles, mg sin theta behaves approximately like mg theta, leading to simple harmonic motion with period T ≈ 2π√(L/g).

The restoring force comes from gravity acting along the path of the bob back toward the lowest point. When the bob is displaced by an angle theta, its weight mg has a tangential component mg sin theta that pulls the bob back down the arc. This tangential component is what drives the back-and-forth motion toward equilibrium.

The tension in the cord points along the string toward the pivot and is purely radial. It doesn’t have a tangential component to pull the bob toward the bottom, so it doesn’t act as a restoring force. It mainly provides the inward (centripetal) force needed to keep the bob moving along its circular path. Friction is neglected in the ideal model, so it doesn’t contribute to restoring either.

Thus gravity’s tangential component is the primary restoring force. For small angles, mg sin theta behaves approximately like mg theta, leading to simple harmonic motion with period T ≈ 2π√(L/g).