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Ch 15: Oscillations
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 15, Problem 78a

A uniform rod of length L oscillates as a pendulum about a pivot that is a distance x from the center. For what value of x, in terms of L, is the oscillation period a minimum?

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Start by recognizing that the problem involves a physical pendulum, where the period of oscillation depends on the moment of inertia about the pivot point and the distance from the pivot to the center of mass.
The period of a physical pendulum is given by the formula: T=2πImgd, where I is the moment of inertia about the pivot, m is the mass of the rod, g is the acceleration due to gravity, and d is the distance from the pivot to the center of mass.
The moment of inertia of the rod about the pivot point is given by the parallel axis theorem: I=Icm+mx2, where Icm is the moment of inertia about the center of mass, and x is the distance from the pivot to the center of mass. For a uniform rod, Icm=112mL2.
Substitute I and d into the period formula. The distance d is related to x as d=x. The period becomes a function of x: T=2π112mL2+mx2mgx.
To find the value of x that minimizes the period, take the derivative of T with respect to x, set it equal to zero, and solve for x. This involves applying calculus to the expression for T and simplifying. The result will yield the value of x in terms of L.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Pendulum Motion

Pendulum motion refers to the oscillatory movement of a mass attached to a rod or string that swings back and forth under the influence of gravity. The period of a simple pendulum is influenced by its length and the acceleration due to gravity, with the formula T = 2π√(L/g). In this case, the uniform rod's pivot point affects the effective length of the pendulum, thus altering its oscillation period.
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Moment of Inertia

The moment of inertia is a measure of an object's resistance to changes in its rotational motion. For a rod pivoting about a point, the moment of inertia depends on the mass distribution relative to the pivot point. The formula for the moment of inertia of a uniform rod about an end is I = (1/3)mL², and this affects the dynamics of the pendulum's oscillation, influencing the period.
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Minimum Period Condition

The minimum period condition in pendulum motion occurs when the effective length of the pendulum is optimized to reduce the oscillation period. By analyzing the relationship between the pivot point and the center of mass of the rod, one can derive the specific distance x from the center that minimizes the period. This involves calculus and the principles of dynamics to find the critical point where the period is at its lowest.
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Related Practice
Textbook Question

The greenhouse-gas carbon dioxide molecule CO₂ strongly absorbs infrared radiation when its vibrational normal modes are excited by light at the normal-mode frequencies. CO₂ is a linear triatomic molecule, as shown in FIGURE CP15.82, with oxygen atoms of mass mo bonded to a central carbon atom of mass mc. You know from chemistry that the atomic masses of carbon and oxygen are, respectively, 12 and 16. Assume that the bond is an ideal spring with spring constant k. There are two normal modes of this system for which oscillations take place along the axis. (You can ignore additional bending modes.) In this problem, you will find the normal modes and then use experimental data to determine the bond spring constant. The symmetric stretch frequency is known to be 4.00 X 10¹³ Hz. What is the spring constant of the C - O bond? Use 1 u = 1 atomic mass unit = 1.66 X 10⁻²⁷ kg to find the atomic masses in SI units. Interestingly, the spring constant is similar to that of springs you might use in the lab.

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Textbook Question

A spring is standing upright on a table with its bottom end fastened to the table. A block is dropped from a height 3.0 cm above the top of the spring. The block sticks to the top end of the spring and then oscillates with an amplitude of 10 cm. What is the oscillation frequency?

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Textbook Question

The greenhouse-gas carbon dioxide molecule CO₂ strongly absorbs infrared radiation when its vibrational normal modes are excited by light at the normal-mode frequencies. CO₂ is a linear triatomic molecule, as shown in FIGURE CP15.82, with oxygen atoms of mass mo bonded to a central carbon atom of mass mc. You know from chemistry that the atomic masses of carbon and oxygen are, respectively, 12 and 16. Assume that the bond is an ideal spring with spring constant k. There are two normal modes of this system for which oscillations take place along the axis. (You can ignore additional bending modes.) In this problem, you will find the normal modes and then use experimental data to determine the bond spring constant. Use the frequency of the symmetric stretch to predict the frequency of the antisymmetric stretch. The measured frequency is 7.05 × 1013 Hz so your prediction is close but not perfect. The reason is that the bonds are not ideal springs but have a slight amount of anharmonicity. Nonetheless, you’ve learned a great deal about the CO₂ molecule from a simple model of oscillating masses.

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Textbook Question

A 200 g oscillator in a vacuum chamber has a frequency of 2.0 Hz. When air is admitted, the oscillation decreases to 60% of its initial amplitude in 50 s. How many oscillations will have been completed when the amplitude is 30% of its initial value?

Textbook Question

Suppose a large spherical object, such as a planet, with radius R and mass M has a narrow tunnel passing diametrically through it. A particle of mass m is inside the tunnel at a distance 𝓍 ≤ R from the center. It can be shown that the net gravitational force on the particle is due entirely to the sphere of mass with radius 𝓇 ≤ 𝓍 there is no net gravitational force from the mass in the spherical shell with 𝓇 > 𝓍. a. Find an expression for the gravitational force on the particle, assuming the object has uniform density. Your expression will be in terms of x, R, m, M, and any necessary constants.

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Textbook Question

A block on a frictionless table is connected as shown in FIGURE P15.75 to two springs having spring constants k₁ and k₂. Find an expression for the block’s oscillation frequency f in terms of the frequencies f₁ and f₂ at which it would oscillate if attached to spring 1 or spring 2 alone.