A mass hanging from a spring oscillates with a period of 0.35 s. Suppose the mass and spring are swung in a horizontal circle, with the free end of the spring at the pivot. What rotation frequency, in rpm, will cause the spring's length to stretch by 15%?
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17. Periodic Motion
Intro to Simple Harmonic Motion (Horizontal Springs)
Problem 16h
Textbook Question
A 200 g mass attached to a horizontal spring oscillates at a frequency of 2.0 Hz. At t = 0 s, the mass is at x = 5.0 cm and has vx = -30 cm/s. Determine: The position at t = 0.40 s.

1
Convert all given quantities to SI units: mass (m) = 200 g = 0.2 kg, frequency (f) = 2.0 Hz, initial position (x₀) = 5.0 cm = 0.05 m, and initial velocity (vₓ₀) = -30 cm/s = -0.30 m/s.
Determine the angular frequency (ω) of the oscillation using the formula: . Substitute f = 2.0 Hz to calculate ω.
Write the general equation for the position of a mass in simple harmonic motion: , where A is the amplitude and ϕ is the phase constant. Use the initial conditions to determine A and ϕ.
Use the initial position (x₀ = 0.05 m) and initial velocity (vₓ₀ = -0.30 m/s) to solve for A and ϕ. Start by substituting x₀ into the position equation and vₓ₀ into the velocity equation: . Solve the system of equations to find A and ϕ.
Substitute the values of A, ω, and ϕ into the position equation . Evaluate the position at t = 0.40 s to find x(0.40 s).

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Simple Harmonic Motion (SHM)
Simple Harmonic Motion is a type of periodic motion where an object oscillates around an equilibrium position. The motion is characterized by a restoring force proportional to the displacement from the equilibrium, leading to sinusoidal position and velocity functions over time. In this case, the mass-spring system exhibits SHM, which can be described using parameters like amplitude, frequency, and phase.
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Frequency and Angular Frequency
Frequency is the number of oscillations per unit time, measured in Hertz (Hz). Angular frequency, denoted by ω, relates to frequency through the equation ω = 2πf, where f is the frequency. In this problem, the frequency of 2.0 Hz indicates how quickly the mass oscillates, which is crucial for determining its position at any given time.
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Position and Velocity in SHM
In SHM, the position (x) and velocity (v) of the oscillating mass can be expressed as functions of time. The position can be described by the equation x(t) = A cos(ωt + φ), where A is the amplitude, ω is the angular frequency, and φ is the phase constant. The velocity is the derivative of the position function, indicating how the position changes over time, which is essential for calculating the position at a specific time.
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