THE PRACTICE STUDIO
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PRACTICE / Damped oscillations
Infer damping from peak energies
For a freely decaying linear oscillator, two turning points on the same side are separated by four complete damped cycles and 8.0 s. Their spring energies are 2.0 mJ and 0.50 mJ. The damping force is proportional to velocity.
What amplitude decay rate γ is consistent with these data?
Organize the givens & assumptions
- Same-side peak energies: 2.0 mJ and 0.50 mJ
- Elapsed time: 8.0 s
- Amplitude envelope proportional to exp(−γt)
Constant mass, spring stiffness and positive viscous damping. Compare equal-phase turning points, where all mechanical energy is spring energy; do not assume instantaneous total energy is a pure exponential at every phase.
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Problem: Infer damping from peak energies
For a freely decaying linear oscillator, two turning points on the same side are separated by four complete damped cycles and 8.0 s. Their spring energies are 2.0 mJ and 0.50 mJ. The damping force is proportional to velocity.
Question: What amplitude decay rate γ is consistent with these data?
Givens:
- Same-side peak energies: 2.0 mJ and 0.50 mJ
- Elapsed time: 8.0 s
- Amplitude envelope proportional to exp(−γt)
Assumptions: Constant mass, spring stiffness and positive viscous damping. Compare equal-phase turning points, where all mechanical energy is spring energy; do not assume instantaneous total energy is a pure exponential at every phase.
Choices (LaTeX):
A. \frac{\ln4}{8}\ \mathrm{s^{-1}}
B. \frac{\ln2}{2}\ \mathrm{s^{-1}}
C. \frac14\ \mathrm{s^{-1}}
D. \frac{\ln2}{8}\ \mathrm{s^{-1}}
Ask for my approach first. Help me distinguish a physical misconception from an algebra error without revealing which choice is correct.
Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.Start with the smallest hint.
Work through it with me.
One decision at a time. Pause, predict, then unpack the algebra.
An energy ratio and an amplitude ratio have different decay exponents.
Use the turning-point store
At each selected turning point velocity is zero, so the measured spring energies determine the displacement-peak ratio.
Why is this step valid?
This equal-phase ratio is exact for linear underdamped motion.
Fit the amplitude exponent
Use the actual elapsed time rather than treating the cycle count as a time.
Why is this step valid?
The decay constant has units of inverse seconds, so a time interval is required.
Interpret the result
The inferred rate corresponds to an amplitude-envelope e-folding time of about 11.54 s.
Why is this step valid?
An energy-envelope timescale would be one half this amplitude timescale.
Squaring the inferred equal-phase amplitude factor reproduces the measured energy change.
TRY THE IDEA AGAIN
Do two arbitrary signed displacements at different phases directly give the exponential envelope ratio?
YOUR LEARNING NOTE
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