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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 63

A 500 g air-track glider attached to a spring with spring constant 10 N/m is sitting at rest on a frictionless air track. A 250 g glider is pushed toward it from the far end of the track at a speed of 120 cm/s. It collides with and sticks to the 500 g glider. What are the amplitude and period of the subsequent oscillations?

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Convert all given quantities to SI units. The mass of the first glider is \( m_1 = 0.500 \; \text{kg} \), the mass of the second glider is \( m_2 = 0.250 \; \text{kg} \), the spring constant is \( k = 10 \; \text{N/m} \), and the velocity of the second glider is \( v_2 = 1.20 \; \text{m/s} \).
Determine the total mass of the system after the collision. Since the two gliders stick together, the combined mass is \( M = m_1 + m_2 \).
Use the principle of conservation of momentum to find the velocity of the combined gliders immediately after the collision. The total momentum before the collision is \( p_{\text{initial}} = m_2 \cdot v_2 \), and the total momentum after the collision is \( p_{\text{final}} = M \cdot v_{\text{final}} \). Solve for \( v_{\text{final}} \) using \( p_{\text{initial}} = p_{\text{final}} \).
Determine the amplitude of oscillation. The kinetic energy of the combined gliders immediately after the collision is converted into the potential energy of the spring at maximum compression. Use the equation \( \frac{1}{2} M v_{\text{final}}^2 = \frac{1}{2} k A^2 \), where \( A \) is the amplitude. Solve for \( A \).
Calculate the period of oscillation using the formula for the period of a mass-spring system: \( T = 2 \pi \sqrt{\frac{M}{k}} \), where \( M \) is the total mass of the system and \( k \) is the spring constant.

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Key Concepts

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

Conservation of Momentum

In a closed system, the total momentum before a collision is equal to the total momentum after the collision. This principle is crucial for analyzing the collision between the two gliders, as it allows us to determine their combined velocity immediately after they stick together.
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Simple Harmonic Motion (SHM)

SHM describes the oscillatory motion of an object when it is displaced from its equilibrium position and experiences a restoring force proportional to that displacement. The gliders will undergo SHM after the collision due to the restoring force provided by the spring, which is characterized by its spring constant.
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Amplitude and Period of Oscillation

The amplitude of oscillation is the maximum displacement from the equilibrium position, while the period is the time taken to complete one full cycle of motion. For the combined mass of the gliders and the spring constant, these parameters can be calculated using the formulas for a mass-spring system, which relate mass, spring constant, and energy conservation.
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Related Practice
Textbook Question

A uniform rod of mass M and length L swings as a pendulum on a pivot at distance L/4 from one end of the rod. Find an expression for the frequency f of small-angle oscillations.

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Textbook Question

It has recently become possible to 'weigh' DNA molecules by measuring the influence of their mass on a nano-oscillator. FIGURE P15.58 shows a thin rectangular cantilever etched out of silicon (density 2300 kg/m³) with a small gold dot (not visible) at the end. If pulled down and released, the end of the cantilever vibrates with SHM, moving up and down like a diving board after a jump. When bathed with DNA molecules whose ends have been modified to bind with gold, one or more molecules may attach to the gold dot. The addition of their mass causes a very slight—but measurable—decrease in the oscillation frequency. A vibrating cantilever of mass M can be modeled as a block of mass ⅓M attached to a spring. (The factor of ⅓ arises from the moment of inertia of a bar pivoted at one end.) Neither the mass nor the spring constant can be determined very accurately—perhaps to only two significant figures—but the oscillation frequency can be measured with very high precision simply by counting the oscillations. In one experiment, the cantilever was initially vibrating at exactly 12 MHz. Attachment of a DNA molecule caused the frequency to decrease by 50 Hz. What was the mass of the DNA?

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Textbook Question

The 15 g head of a bobble-head doll oscillates in SHM at a frequency of 4.0 Hz. The amplitude of the head's oscillations decreases to 0.5 cm in 4.0 s. What is the head's damping constant?

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Textbook Question

A 15-cm-long, 200 g rod is pivoted at one end. A 20 g ball of clay is stuck on the other end. What is the period if the rod and clay swing as a pendulum?

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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?

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Textbook Question

Interestingly, there have been several studies using cadavers to determine the moments of inertia of human body parts, information that is important in biomechanics. In one study, the center of mass of a 5.0 kg lower leg was found to be 18 cm from the knee. When the leg was allowed to pivot at the knee and swing freely as a pendulum, the oscillation frequency was 1.6 Hz. What was the moment of inertia of the lower leg about the knee joint?

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