What causes the restoring force in a simple pendulum?


A. friction between the thread and the support

B. the force exerted on the pendulum bob while displacing it from the mean position

C. the component of the weight of the bob acting perpendicular to the thread

D. the component of the weight of the bob acting parallel to the thread

gravity wants to make the pendenlum bob fall, the restoring force is the force exerting on the bob while it is displaced from the mean position.

The restoring force in a simple pendulum is caused by the component of the weight of the bob acting perpendicular to the thread. To understand why this is the case, let's go through the options and examine each one:

A. Friction between the thread and the support: Friction does exist in a pendulum but it does not cause the restoring force. Friction only opposes the motion and causes the pendulum to eventually come to a stop.

B. The force exerted on the pendulum bob while displacing it from the mean position: While a force is indeed exerted on the pendulum bob when it is displaced from the mean position, this force is not the restoring force. This force is responsible for causing the pendulum to deviate from its equilibrium position.

C. The component of the weight of the bob acting perpendicular to the thread: This is the correct answer. When the pendulum bob is displaced from its equilibrium position, gravity acts on it and creates a force that is decomposed into two components: one component acts tangentially to the circular path of the pendulum, which does not contribute to the restoring force, and the other component acts perpendicular to the thread, which is responsible for the restoring force that brings the pendulum back to its equilibrium position.

D. The component of the weight of the bob acting parallel to the thread: This component of the weight acts tangentially to the circular path of the pendulum and does not contribute to the restoring force.

In summary, option C, the component of the weight of the bob acting perpendicular to the thread, is what causes the restoring force in a simple pendulum.