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/mยณ) 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 slightโbut measurableโdecrease in the oscillation frequency. A vibrating cantilever of mass M can be modeled as a block of mass โ M attached to a spring. (The factor of โ arises from the moment of inertia of a bar pivoted at one end.) Neither the mass nor the spring constant can be determined very accuratelyโperhaps to only two significant figuresโbut 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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17. Periodic Motion
Intro to Simple Harmonic Motion (Horizontal Springs)
Problem 17e
Textbook Question
The position of a 50 g oscillating mass is given by ๐(t) = (2.0 cm) cos (10 t - ฯ/4), where t is in s. Determine: The initial conditions.

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Step 1: Understand the given equation for the position of the oscillating mass: ๐(t) = (2.0 cm) cos(10t - ฯ/4). Here, the amplitude is 2.0 cm, the angular frequency is 10 rad/s, and the phase constant is -ฯ/4. The initial conditions refer to the position and velocity of the mass at t = 0.
Step 2: To find the initial position, substitute t = 0 into the position equation. This gives ๐(0) = (2.0 cm) cos(-ฯ/4). Use the trigonometric identity cos(-ฮธ) = cos(ฮธ) to simplify the expression.
Step 3: To find the initial velocity, differentiate the position equation with respect to time to get the velocity equation. The derivative of ๐(t) = (2.0 cm) cos(10t - ฯ/4) is v(t) = -Aฯ sin(ฯt - ฯ), where A is the amplitude, ฯ is the angular frequency, and ฯ is the phase constant. Substituting the given values, v(t) = -(2.0 cm)(10 rad/s) sin(10t - ฯ/4).
Step 4: Substitute t = 0 into the velocity equation to find the initial velocity. This gives v(0) = -(2.0 cm)(10 rad/s) sin(-ฯ/4). Use the trigonometric identity sin(-ฮธ) = -sin(ฮธ) to simplify the expression.
Step 5: Combine the results from Step 2 and Step 4 to summarize the initial conditions: the initial position ๐(0) and the initial velocity v(0). These values describe the state of the oscillating mass at t = 0.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Oscillation
Oscillation refers to the repetitive variation, typically in time, of some measure about a central value or between two or more different states. In this context, it describes the motion of the mass as it moves back and forth around an equilibrium position, characterized by parameters such as amplitude, frequency, and phase.
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Initial Conditions
Initial conditions are the values of the variables at the start of the observation or experiment, which in this case refers to the position and velocity of the oscillating mass at time t = 0. These conditions are crucial for determining the future behavior of the system and are derived from the given position function.
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Example
Phase Angle
The phase angle in oscillatory motion indicates the initial position of the oscillating object relative to a reference point. In the equation provided, the phase angle of -ฯ/4 affects the starting position of the mass, shifting the cosine function and thus altering the initial conditions of the oscillation.
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Phase Constant of a Wave Function
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