Should the Answer Be B? | Reasoning Explained

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The discussion centers on the reasoning behind selecting answer B instead of E in a physics problem related to simple harmonic motion (SHM). It emphasizes the correct application of formulas for the period of a simple pendulum and a mass on a spring, highlighting the confusion that arises from mixing these equations. The distinction between angular frequency and angular speed is clarified, noting that while both use the symbol ω, they represent different concepts in SHM and circular motion. The conversation encourages visualizing a pendulum's motion to understand the variability of angular speed during SHM. Ultimately, the explanation successfully clarifies the reasoning for the answer choice.
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Homework Statement
The angular frequency of a simple pendulum depends on its length and on the local acceleration due to gravity. The rate at which the angular displacement of the pendulum changes, dθ/dt, is:

A. √ (mgL/I)
B. √(g/L)
C. 2兀 (L/g)
D. √(k/m)
E. none of the above
Relevant Equations
F=-kx
T=2兀/w
w = √(g/L)
I think the answer should be B, not E.

Here's the reasoning:
F = -kx = mg sinθ = -mg θ (because θ very small) = -mgx/L
T = 2π*√(m/k) = 2π*√(m/(mg/L)) = 2π*√(L/g)
Also, T= 2π/w and thus, w = dθ/dt = √(g/L)
 
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hidemi said:
Here's the reasoning:
F = -kx = mg sinθ = -mg θ (because θ very small) = -mgx/L
T = 2π*√(m/k) = 2π*√(m/(mg/L)) = 2π*√(L/g)
Also, T= 2π/w and thus, w = dθ/dt = √(g/L)
Point 1

For simple harmonic motion (SHM):
The standard equation for the period of a simple pendulum is ##T = 2\pi \sqrt \frac L g##.
The standard equation for the period of a mass (m) oscillating on a spring (spring constant k) is ##T = 2\pi \sqrt\frac m k##

You seem to be mixing these equations together! By accident, this may give a sensible answer - but it is incorrect to do this.

Point 2

Angular frequency and angular speed are not the same thing, though the same symbol, ##\omega##, is often used for both. This causes confusion!

In the context of SHM, angular frequency is given by ##\omega = \frac {2\pi} {T}## where T is the period, Since T is constant for SHM, ##\omega## is constant.

In the context of a changing angle (e.g. circular motion), angular speed is given by ##\omega = \frac {d\theta} {dt}##. It is the rate of change of angular displacement. It doesn't have to be constant.

In the context of a pendulum, angular frequency, ##\omega##, is not the same thing as ## \frac {d\theta} {dt}##

Point 3

Imagine (or even watch!) a pendulum. Do you think ##\frac {d\theta} {dt}## is constant during SHM? Does this help you answer the original question?
 
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Steve4Physics said:
Point 1

For simple harmonic motion (SHM):
The standard equation for the period of a simple pendulum is ##T = 2\pi \sqrt \frac L g##.
The standard equation for the period of a mass (m) oscillating on a spring (spring constant k) is ##T = 2\pi \sqrt\frac m k##

You seem to be mixing these equations together! By accident, this may give a sensible answer - but it is incorrect to do this.

Point 2

Angular frequency and angular speed are not the same thing, though the same symbol, ##\omega##, is often used for both. This causes confusion!

In the context of SHM, angular frequency is given by ##\omega = \frac {2\pi} {T}## where T is the period, Since T is constant for SHM, ##\omega## is constant.

In the context of a changing angle (e.g. circular motion), angular speed is given by ##\omega = \frac {d\theta} {dt}##. It is the rate of change of angular displacement. It doesn't have to be constant.

In the context of a pendulum, angular frequency, ##\omega##, is not the same thing as ## \frac {d\theta} {dt}##

Point 3

Imagine (or even watch!) a pendulum. Do you think ##\frac {d\theta} {dt}## is constant during SHM? Does this help you answer the original question?
Thank you. I got it.
 
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