Frequency and Period of SHM

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AP Physics C: Mechanics › Frequency and Period of SHM

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1

Using the given parameters, a pendulum of length $L=2.0,\text{m}$ swings at small angles; determine the time for one full oscillation.

$T=\pi\sqrt{\dfrac{L}{g}},\text{s}$

$T=2\pi\sqrt{\dfrac{g}{L}},\text{s}$

$T=2\pi\sqrt{\dfrac{L}{g}},\text{s}$

$T=2\pi\sqrt{\dfrac{m}{k}},\text{s}$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a simple pendulum at small angles, the period T is determined by T = 2π√(L/g), where L is the pendulum length and g is gravitational acceleration. Choice B is correct because it shows the standard pendulum formula T = 2π√(L/g), which with L = 2.0 m and g = 9.8 m/s² gives T = 2π√(2.0/9.8) = 2.84 s. Choice A is incorrect because it's missing the factor of 2 in front of π, which would give half the actual period. To help students: Derive the pendulum formula from the restoring torque τ = -mgL sin θ ≈ -mgLθ for small angles. Practice calculating periods for different pendulum lengths to build intuition about the √L dependence.

2

Based on the described system, a $0.80,\text{kg}$ mass on a $k=50,\text{N/m}$ spring oscillates; calculate the frequency in Hz.

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{k}{m}},\text{Hz}$

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{g}{L}},\text{Hz}$

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{m}{k}},\text{Hz}$

$f=2\pi\sqrt{\dfrac{k}{m}},\text{Hz}$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a mass-spring system, the frequency f is determined by f = (1/2π)√(k/m), where k is the spring constant and m is mass. Choice A is correct because it properly shows f = (1/2π)√(k/m), which with k = 50 N/m and m = 0.80 kg gives f = (1/2π)√(50/0.80) = 1.26 Hz. Choice C is incorrect because it inverts the k/m ratio, resulting in incorrect units and a physically meaningless result. To help students: Connect frequency to the period formula by showing f = 1/T = 1/(2π√(m/k)) = (1/2π)√(k/m). Use unit analysis to verify that √(k/m) has units of s⁻¹, making the frequency have proper Hz units.

3

Based on the described system, a lightly damped oscillator with $m=1.0,\text{kg}$ and $k=100,\text{N/m}$ has small damping; what best describes its period?

Approximately $T\approx 2\pi\sqrt{m/k}$; damping mainly reduces amplitude

Period becomes $T=2\pi\sqrt{k/m}$ because damping reverses the ratio

Period decreases to $T=\dfrac{1}{2\pi}\sqrt{m/k}$ due to damping

Exactly $T=2\pi\sqrt{m/k}$ is impossible; damping makes $T$ undefined

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a lightly damped oscillator, the period remains approximately T ≈ 2π√(m/k), with damping primarily affecting amplitude decay rather than period. Choice A is correct because light damping causes exponential amplitude decay while leaving the period nearly unchanged - the actual period is slightly longer but the difference is negligible for light damping. Choice B is incorrect because damping doesn't make period undefined; the system still oscillates with a well-defined period. To help students: Introduce the damped oscillator equation and show that for light damping (ζ << 1), the period T ≈ T₀ = 2π√(m/k). Use energy arguments to explain why damping reduces amplitude but minimally affects frequency.

4

Using the given parameters, a $0.50,\text{kg}$ block on a $k=200,\text{N/m}$ spring oscillates; what is the period?

$T=\dfrac{1}{2\pi}\sqrt{\dfrac{m}{k}},\text{s}$

$T=2\pi\sqrt{\dfrac{L}{g}},\text{s}$

$T=2\pi\sqrt{\dfrac{m}{k}},\text{s}$

$T=2\pi\sqrt{\dfrac{k}{m}},\text{s}$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a mass-spring system, the period T is determined by T = 2π√(m/k), where m is mass and k is the spring constant. Choice B is correct because it shows the proper formula T = 2π√(m/k), which when calculated with m = 0.50 kg and k = 200 N/m gives T = 2π√(0.50/200) = 0.314 s. Choice A is incorrect because it inverts the mass and spring constant ratio, which would give units of s⁻¹ rather than seconds. To help students: Emphasize dimensional analysis to verify that √(m/k) has units of time. Practice deriving the period formula from F = -kx and Newton's second law to build deeper understanding.

5

Using the given parameters, a horizontal mass-spring oscillator has $m=0.40,\text{kg}$, $k=160,\text{N/m}$, amplitude $0.05,\text{m}$, and $v(0)=0$ at $x(0)=+A$; how does the period change if the mass is doubled?

It halves: $T' = \tfrac{1}{2}T$.

It is unchanged: $T' = T$.

It increases by $\sqrt{2}$: $T' = \sqrt{2},T$.

It doubles: $T' = 2T$.

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a mass-spring system, the period T is determined by T = 2π√(m/k), where m is mass and k is the spring constant. Choice C is correct because when mass doubles (m' = 2m), the new period becomes T' = 2π√(2m/k) = √2 × 2π√(m/k) = √2 × T, showing that period increases by a factor of √2. Choice A is incorrect because it assumes a linear relationship between mass and period, when the actual relationship involves a square root. To help students: Emphasize that period depends on √m, not m directly. Practice problems where system parameters change to reinforce how T scales with √(m/k).

6

Based on the described system, a simple pendulum of length $0.80,\text{m}$ is released from rest at $6^\circ$; using $T=2\pi\sqrt{L/g}$, what is the frequency in Hz?

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{g}{L}}$

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{L}{g}}$

$f=2\pi\sqrt{\dfrac{L}{g}}$

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{k}{m}}$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a simple pendulum, the period T is given by T = 2π√(L/g), where L is length and g is gravitational acceleration. Choice A is correct because it properly converts period to frequency using f = 1/T = 1/(2π√(L/g)) = (1/2π)√(g/L), showing understanding of the inverse relationship. Choice B is incorrect because it represents the period formula, not frequency, demonstrating confusion between these reciprocal quantities. To help students: Emphasize the distinction between period (time per cycle) and frequency (cycles per time). Practice converting between T and f using f = 1/T consistently across different SHM systems.

7

Using the given parameters, a $1.2,\text{kg}$ block on a $k=300,\text{N/m}$ spring oscillates; how does period change if mass doubles?

It decreases by $\sqrt{2}$: $T' = T/\sqrt{2}$

It doubles: $T' = 2T$

It is unchanged: $T' = T$

It increases by $\sqrt{2}$: $T' = \sqrt{2},T$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a mass-spring system, the period T is determined by T = 2π√(m/k), showing that period is proportional to √m. Choice B is correct because when mass doubles from m to 2m, the new period T' = 2π√(2m/k) = √2 × 2π√(m/k) = √2 × T. Choice A is incorrect because it assumes a linear relationship between mass and period, when the actual relationship involves the square root. To help students: Emphasize that T ∝ √m, not T ∝ m, which means doubling mass increases period by √2 ≈ 1.414. Practice problems where students predict how period changes when mass is multiplied by various factors (4, 9, 1/4) to reinforce the square root relationship.

8

Based on the described system, a vertical spring with $m=0.60,\text{kg}$ and $k=240,\text{N/m}$ oscillates about equilibrium; what is the period?

$T=2\pi\sqrt{\dfrac{L}{g}},\text{s}$

$T=2\pi\sqrt{\dfrac{k}{m}},\text{s}$

$T=2\pi\sqrt{\dfrac{mg}{k}},\text{s}$

$T=2\pi\sqrt{\dfrac{m}{k}},\text{s}$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a vertical mass-spring system, the period T is still determined by T = 2π√(m/k), independent of gravity's effect on equilibrium position. Choice A is correct because the period formula T = 2π√(m/k) applies to both horizontal and vertical springs, giving T = 2π√(0.60/240) = 0.314 s. Choice C is incorrect because it incorrectly includes gravity (mg/k), which only affects the equilibrium position, not the period of oscillation. To help students: Emphasize that gravity shifts the equilibrium position but doesn't change the period. Show that the restoring force F = -k(x - x₀) still leads to the same differential equation and period formula.

9

Based on the described system, a small-angle pendulum with $L=1.6,\text{m}$ oscillates; calculate the frequency in Hz using $f=1/T$.

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{g}{L}},\text{Hz}$

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{k}{m}},\text{Hz}$

$f=\dfrac{1}{2\pi}\sqrt{\dfrac{L}{g}},\text{Hz}$

$f=2\pi\sqrt{\dfrac{g}{L}},\text{Hz}$

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a simple pendulum, the period is T = 2π√(L/g), so the frequency f = 1/T = 1/(2π√(L/g)) = (1/2π)√(g/L). Choice A is correct because it properly shows f = (1/2π)√(g/L), which with L = 1.6 m and g = 9.8 m/s² gives f = (1/2π)√(9.8/1.6) = 0.394 Hz. Choice B is incorrect because it inverts the g/L ratio, which would give incorrect units and violate the physics - longer pendulums have lower frequencies. To help students: Show the algebraic manipulation from T to f explicitly. Use dimensional analysis to verify that √(g/L) has units of s⁻¹, confirming the frequency formula is correct.

10

Based on the described system, a $0.60,\text{kg}$ block attached to a spring $k=240,\text{N/m}$ oscillates with amplitude $0.08,\text{m}$; what is the effect on frequency if the spring constant is halved?

Frequency doubles: $f' = 2f$.

Frequency is unchanged: $f' = f$.

Frequency halves: $f' = \tfrac{1}{2}f$.

Frequency decreases by $\sqrt{2}$: $f' = \tfrac{f}{\sqrt{2}}$.

Explanation

This question tests understanding of the frequency and period of simple harmonic motion in AP Physics C: Mechanics. In SHM, the period is the time it takes to complete one full oscillation, and frequency is the number of oscillations per unit time. They are inversely related (f = 1/T). For a mass-spring system, the frequency f is given by f = (1/2π)√(k/m), where k is the spring constant and m is mass. Choice B is correct because when the spring constant is halved (k' = k/2), the new frequency becomes f' = (1/2π)√(k/2m) = (1/√2) × (1/2π)√(k/m) = f/√2, showing frequency decreases by a factor of √2. Choice C is incorrect because it assumes a linear relationship between spring constant and frequency, missing the square root dependence. To help students: Emphasize that frequency depends on √k, not k directly. Use dimensional analysis to verify that √(k/m) has units of 1/time, appropriate for frequency.

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