15: Oscillations
We begin the study of oscillations with simple systems of pendulums and springs. Although these systems may seem quite basic, the concepts involved have many real-life applications.
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- 15.1: Prelude to Oscillations
- The Comcast Building in Philadelphia, Pennsylvania, has a tuned-mass damper is used to reduce the oscillations. Installed at the top of the building is a tuned, liquid-column mass damper, consisting of a 300,000-gallon reservoir of water. This U-shaped tank allows the water to oscillate freely at a frequency that matches the natural frequency of the building. Damping is provided by tuning the turbulence levels in the moving water using baffles.
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- 15.2: Simple Harmonic Motion
- A very common type of periodic motion is called simple harmonic motion (SHM). A system that oscillates with SHM is called a simple harmonic oscillator. In simple harmonic motion, the acceleration of the system, and therefore the net force, is proportional to the displacement and acts in the opposite direction of the displacement.
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- 15.3: Energy in Simple Harmonic Motion
- The simplest type of oscillations are related to systems that can be described by Hooke’s law, F = −kx, where F is the restoring force, x is the displacement from equilibrium or deformation, and k is the force constant of the system. Elastic potential energy stored in the deformation of a system can be described by Hooke’s law as U = (1/2)kx^2. Energy in the simple harmonic oscillator is shared between elastic potential energy and kinetic energy, with the total being constant.
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- 15.4: Comparing Simple Harmonic Motion and Circular Motion
- A projection of uniform circular motion undergoes simple harmonic oscillation. Consider a circle with a radius A, moving at a constant angular speed ω. A point on the edge of the circle moves at a constant tangential speed of v_max = Aω. The projection of the radius onto the x-axis is x(t) = Acos(ωt + ϕ), where (ϕ) is the phase shift. The x-component of the tangential velocity is v(t) = −Aωsin(ωt + ϕ).
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- 15.5: Pendulums
- A mass m suspended by a wire of length L and negligible mass is a simple pendulum and undergoes SHM for amplitudes less than about 15°. The period of a simple pendulum is T = 2π√Lg, where L is the length of the string and g is the acceleration due to gravity. The period of a physical pendulum can be found if the moment of inertia is known where the length between the point of rotation and the center of mass is L.
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- 15.6: Damped Oscillations
- Damped harmonic oscillators have non-conservative forces that dissipate their energy. Critical damping returns the system to equilibrium as fast as possible without overshooting. An underdamped system will oscillate through the equilibrium position. An overdamped system moves more slowly toward equilibrium than one that is critically damped.
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- 15.7: Forced Oscillations
- A system’s natural frequency is the frequency at which the system oscillates if not affected by driving or damping forces. A periodic force driving a harmonic oscillator at its natural frequency produces resonance. The system is said to resonate. The less damping a system has, the higher the amplitude of the forced oscillations near resonance. The more damping a system has, the broader response it has to varying driving frequencies.
Thumbnail: A picture of the first Tacoma Narrows Bridge. The 1940 Tacoma Narrows Bridge, the first Tacoma Narrows Bridge, was a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It dramatically collapsed into Puget Sound on November 7 of the same year.