PEMBAHASAN SOAL OSN IPA SMP TAHUN 2025 | PART 2

Gelombang (Wave Mechanics)
📌 A wave travels along a 2 m long string at a speed ($v$) of $20 \text{ m/s}$.
🌊 Given a frequency ($f$) of $10 \text{ Hz}$, the wavelength ($\lambda$) is calculated using $v = \lambda \times f$.
📏 The resulting wavelength ($\lambda$) is $20 \text{ m/s} / 10 \text{ Hz} =$2 \text{ m}.

Alat Optik (Optical Instruments - Vision Correction)
👓 An individual can read clearly only at $50 \text{ cm}$, while the normal near point ($S_n$) is specified as $30 \text{ cm}$ in the problem context.
👁️ This condition indicates hypermetropia (farsightedness) because the near point is greater than normal ($50 \text{ cm} > 30 \text{ cm}$).
➕ Hypermetropia is corrected using a positive (convex) lens.
🧮 The lens power ($P$) is calculated by $P = \frac{100}{S_n} - \frac{100}{PP_{\text{orang}}}$, yielding $P = \frac{100}{30} - \frac{100}{50} = \frac{10}{3} - 2 = \frac{4}{3}$.
💡 The required lens power is $+1.33 \text{ diopters}$ (when rounded to two decimal places).

Induksi Elektromagnetik (Electromagnetic Induction - Faraday's Law)
🧲 A coil with $N = 400$ turns experiences a magnetic flux ($\Phi$) change from $0.6 \text{ Weber}$ to $1 \text{ Weber}$ over a time interval ($\Delta t$) of $0.2 \text{ seconds}$.
⚡ The change in flux ($\Delta \Phi$) is $1 - 0.6 = 0.4 \text{ Weber}$.
📉 The induced electromotive force (EMF or $\text{E}$) is found using Faraday's Law (ignoring the negative sign based on Lenz's Law for magnitude): $E = N \frac{|\Delta \Phi|}{\Delta t}$.
⚡ The induced $\text{E}$ is $400 \times \frac{0.4}{0.2} = 400 \times 2 =$800 \text{ volts}.

Hukum Coulomb (Coulomb's Law - Electrostatics)
⚖️ Two charged particles exert an attractive force ($F_1$) when separated by a distance $R$, such that $F_1 \propto 1/R^2$.
🤏 If the distance between the charges is increased to $3R$ (resulting in force $F_2$), the new force will be $F_2 \propto 1/(3R)^2$.
⚖️ By comparison, the new force $F_2$ is $\frac{1}{(3)^2}$ times the original force $F_1$.
💥 The final attractive force becomes $F_2 = F_1 / 9$, or $\frac{1}{9}F$.

Key Points & Insights
➡️ The wave speed formula $v = \lambda \times f$ is fundamental for solving basic wave propagation problems.
➡️ For vision correction, identify the defect (hypermetropia) when the observed near point ($50 \text{ cm}$) is further than the standard normal near point ($30 \text{ cm}$).
➡️ Faraday's law requires calculating the change in flux ($\Delta \Phi$) and dividing by the time interval ($\Delta t$), then multiplying by the number of turns ($N$).
➡️ Coulomb's Law shows that electrostatic force is inversely proportional to the square of the distance ($F \propto 1/r^2$); doubling the distance quarters the force.

📸 Video summarized with SummaryTube.com on Feb 08, 2026, 02:14 UTC