Sphere

Calculus Level 2

The volume of a sphere increases at the rate of 2 cm 3 ^3 per second. Find the rate at which the surface area increases when the radius is 3 cm.

1 3 \frac{1}{3} π π c m 2 s cm^2s ^-1 4 3 \frac{4}{3} π π c m 2 s cm^2s ^-1 2 3 \frac{2}{3} c m 2 s cm^2s ^-1 4 3 \frac{4}{3} c m 2 s cm^2s ^-1 1 3 \frac{1}{3} c m 2 s cm^2s ^-1

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2 solutions

Marta Reece
Jun 1, 2017

V = 4 3 π r 3 V=\frac43\pi r^3

d V d t = 4 π r 2 d r d t \dfrac {dV}{dt}=4\pi r^2\dfrac{dr}{dt}

V ( 3 ) = 4 π × 9 d r d t = 2 V'(3)=4\pi \times9\dfrac{dr}{dt}=2

d r d t = 1 18 π \dfrac{dr}{dt}=\dfrac{1}{18\pi}

S = 4 π r 2 S=4\pi r^2

d S d t = 8 π r d r d t \dfrac{dS}{dt}=8\pi r \dfrac{dr}{dt}

S ( 3 ) = 8 π × 3 × 1 18 π = 4 3 S'(3)=8\pi\times 3\times\dfrac{1}{18\pi}=\boxed{\dfrac43}

Nikhil Raj
Jun 1, 2017

Volume of sphere = 4 3 π r 3 d V d t = 4 π r 2 d r d t 2 = 4 π r 2 d r d t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( d V d t is given ) d r d t = 1 2 π r 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( 1 ) Now, Surface area = 4 π r 2 d S d t = 8 π r d r d t d S d t = 8 π r 2 π r 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( b y ( 1 ) ) d S d t = 4 r d S d t r = 3 c m = 4 3 c m 2 / s e c {\text{Volume of sphere}} = \dfrac{4}{3} \pi r^3 \\ \dfrac{dV}{dt} = 4 \pi r^2 \cdot \dfrac{dr}{dt} \\ \implies 2 = 4 \pi r^2 \dfrac{dr}{dt} ...........................................(\because \dfrac{dV}{dt} {\text{is given}}) \\ \implies \dfrac{dr}{dt} = \dfrac{1}{2 \pi r^2} ...........................................(1) \\ {\text{Now, Surface area}} = 4 \pi r^2 \\ \implies \dfrac{dS}{dt} = 8 \pi r \cdot \dfrac{dr}{dt} \\ \implies \dfrac{dS}{dt} = \dfrac{8 \pi r}{2 \pi r^2}...........................................(by (1)) \\ \implies \dfrac{dS}{dt} = \dfrac{4}{r} \\ \dfrac{dS}{dt}_{r = 3 cm} = \color{#EC7300}{\boxed{\dfrac{4}{3} cm^2/sec}}

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