A spherical part is designed with radius \(10.00\,\text{cm}\). Its volume is \(V(r)=\frac{4}{3}\pi r^3\).
a) If the manufactured radius can differ from the design by as much as \(0.03\,\text{cm}\), use a linear approximation to estimate the maximum resulting volume error.
b) A quality standard allows an estimated volume error of at most \(20\,\text{cm}^3\). According to the same linear approximation, what is the largest allowable radius error?
Hints
- Linearize the volume with respect to radius at the design radius.
- For an error bound, use magnitudes of the small input and output changes.
- In part b, work backward from the allowed output error to the permitted input error.
Solution
1. \(V'(r)=4\pi r^2\), so at \(r=10\), \(V'(10)=400\pi\).
2. For a small radius error \(\Delta r\), the volume error satisfies \(|\Delta V|\approx400\pi|\Delta r|\).
3. With \(|\Delta r|=0.03\), \(|\Delta V|\approx12\pi\,\text{cm}^3\approx37.70\,\text{cm}^3\).
4. For the quality standard, require \(400\pi|\Delta r|\le20\). Thus, \(|\Delta r|\le\frac{1}{20\pi}\,\text{cm}\approx0.0159\,\text{cm}\).
Answer
a) Approximately \(12\pi\,\text{cm}^3\approx37.70\,\text{cm}^3\).
b) At most \(\frac{1}{20\pi}\,\text{cm}\approx0.0159\,\text{cm}\).