A spherical body of radius r and density
sigma$\sigma$ falls freely through a viscous liquid having density
rho$\rho$ and viscosity
eta$\eta$ and attains a terminal velocity
v_0$v_{0}$.
Estimated maximum error in the quantity eta$\eta$ is :
(Ignore errors associated with
sigma$\sigma$,
rho$\rho$ and g, gravitational acceleration)
Solution
### Related Formula
v_0 = frac29fracr^2 geta (sigma - rho)$$v_0 = \frac{2}{9}\frac{r^2 g}{\eta} (\sigma - \rho)$$
### Core Logic
Rearranging the formula for viscosity eta$\eta$:
eta = frac29 fracr^2 gv_0 (sigma - rho)$$\eta = \frac{2}{9} \frac{r^2 g}{v_0} (\sigma - \rho)$$
Since frac29$\frac{2}{9}$, g$g$, sigma$\sigma$, and rho$\rho$ are constants or have negligible error, the relative error depends only on r$r$ and v_0$v_0$.
### Step 1: Error Propagation
Taking natural logarithm on both sides:
ln(eta) = ln(textconstant) + 2ln(r) - ln(v_0)$$\ln(\eta) = \ln(\text{constant}) + 2\ln(r) - \ln(v_0)$$
Differentiating for maximum possible error (adding absolute errors):
fracDelta etaeta = 2fracDelta rr + fracDelta v_0v_0$$\frac{\Delta \eta}{\eta} = 2\frac{\Delta r}{r} + \frac{\Delta v_0}{v_0}$$
### Step 2: Final Conclusion
The maximum fractional error is frac2Delta rr + fracDelta v_0v_0$\frac{2\Delta r}{r} + \frac{\Delta v_0}{v_0}$.
### Pattern Recognition
For maximum fractional error of a derived quantity Z = X^a Y^b$Z = X^a Y^b$, the formula is Delta Z/Z = |a|Delta X/X + |b|Delta Y/Y$\Delta Z/Z = |a|\Delta X/X + |b|\Delta Y/Y$. Always sum the absolute relative errors.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 11 Physics: Units and Measurements
Class 11 Physics: Mechanical Properties of Fluids