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15.8: Simple Pendulum

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Simple Pendulum

15.8: Simple Pendulum

A simple pendulum consists of a small diameter ball suspended from a string, which has negligible mass but is strong enough to not stretch. In our daily life, pendulums have many uses, such as in clocks, on a swing set, and on a sinker on a fishing line. 

The period of a simple pendulum depends on two factors: its length and the acceleration due to gravity. The period is completely independent of any other factors, such as mass or maximum displacement. For small displacements, a pendulum is identical to a simple harmonic oscillator, and the period of a pendulum is nearly independent of amplitude, especially if θ is less than approximately 15°. Applying Newton's second law for rotational systems, the equation of motion for a pendulum is obtained.


As an example, consider two simple pendulums suspended from small wires secured to the ceiling of a room. Each pendulum hovers 2 cm above the floor. Pendulum 1 has a bob with a mass of 10 kg. Pendulum 2 has a bob with a mass of 100 kg. Describe how the motion of the pendulums will differ if the bobs are both displaced by 12°.

Since the mass of the bob has no effect on the motion of a simple pendulum, the movement of the pendulums will not differ at all. A pendulum’s motion is only affected by the period (which is related to the pendulum’s length) and by the acceleration due to gravity.

This text is adapted from Openstax, College Physics, Section 16.4: The Simple Pendulum and Openstax, University Physics Volume 1, Section 15.4: Pendulums.

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