Related Formula
The Young's modulus Y$Y$ is:
Y = StressStrain = (F / A)/(Δ l / l) = (F l)/(A Δ l)$$Y = \frac{\text{Stress}}{\text{Strain}} = \frac{F / A}{\Delta l / l} = \frac{F l}{A \Delta l}$$
Solving for extension Δ l$\Delta l$:
Δ l = (F l)/(A Y)$$\Delta l = \frac{F l}{A Y}$$
Since volume V = A l$V = A l$, we substitute l = (V)/(A)$l = \frac{V}{A}$:
Δ l = (F V)/(A² Y)$$\Delta l = \frac{F V}{A^2 Y}$$
Core Logic
We are given that the wires are made of the same material (Y$Y$ is constant) and have the same volume (V$V$ is constant). Also, we want to achieve the same extension (Δ l$\Delta l$ is constant).
Thus, from the formula:
Δ l ∝ (F)/(A²) F ∝ A²$$\Delta l \propto \frac{F}{A^2} \implies F \propto A^2$$
Step 1: Calculate the Force Ratio
Using the proportionality for both wires:
(F₁)/(F₂) = ( (A₁)/(A₂) )²$$\frac{F_1}{F_2} = \left( \frac{A_1}{A_2} \right)^2$$
Given the cross-sectional area ratio is 4:1$4:1$:
(A₁)/(A₂) = (4)/(1)$$\frac{A_1}{A_2} = \frac{4}{1}$$
Substitute this into the ratio equation:
(F₁)/(F₂) = ( (4)/(1) )² = 16$$\frac{F_1}{F_2} = \left( \frac{4}{1} \right)^2 = 16$$
Pattern Recognition
For a constant volume and material, extension Δ l ∝ (F)/(A²)$\Delta l \propto \frac{F}{A^2}$. Since area ratio is 4$4$, the required force ratio is 4² = 16$4^2 = 16$.
Chapter Mix
Class 11 Physics: Mechanical Properties of Solids