単振動と等速円運動

The projection of an object undergoing uniform circular motion along the diameter of its circular path produces simple harmonic motion. As the object rotates at constant angular velocity, its projection oscillates back and forth, exhibiting position, velocity, and acceleration equations identical to those of a simple harmonic oscillator. This relationship demonstrates that simple harmonic motion is fundamentally the one-dimensional shadow of circular motion.

The position of a projection from uniform circular motion can be expressed using either cosine or sine functions, depending on the initial phase angle. At time t, the angle swept is ωt plus the initial phase angle Ф. This mathematical representation captures how the projection oscillates sinusoidally as the object completes its circular path, with the period determined by the circumference divided by velocity.

In uniform circular motion, velocity acts tangentially while acceleration points radially inward. The x-component of both velocity and acceleration equals the corresponding values for the projection. The magnitudes of these components can be derived from standard velocity and acceleration equations, revealing that the projection's motion follows the same kinematic relationships as a simple harmonic oscillator.

The period of the Moon's circular orbit equals the circumference of its orbital path divided by its orbital velocity. This period remains constant throughout the motion since the Moon maintains uniform circular motion at constant angular velocity. The same period applies to the Moon's projection, which undergoes simple harmonic motion with identical temporal characteristics.

Observing the projection of uniform circular motion is often simpler than constructing a precise large-scale simple harmonic oscillator because circular motion naturally produces visible, measurable oscillations. Examples include a ball's shadow on a floor from a rotating turntable or a pen tracing waves on paper beneath a rotating record player. These practical demonstrations make the connection between circular and oscillatory motion immediately apparent.

Hooke's law typically describes systems with simple harmonic motion rather than uniform circular motions at constant angular velocity. While Hooke's law governs the restoring force in oscillatory systems, uniform circular motion involves constant speed and centripetal acceleration. However, the projection of uniform circular motion exhibits forces and accelerations consistent with Hooke's law, linking the two concepts through the projection relationship.

Attach a pen to a rotating turntable or record player and drag paper beneath it to capture the pen's motion as a wave pattern. This wave represents the projection of the circular motion onto a line, visually demonstrating simple harmonic motion. Alternatively, observe the shadow of a ball on a turntable projected onto a floor, which oscillates back and forth as the ball rotates, providing direct evidence of the projection relationship.