Who ate that apple? Dr. Worm ate it.

Calculus Level 4

Two circle graphs of the same radius of 1 intersect each other's center, and the centers are points ( 0.5 , 0 ) (0.5 , 0) & ( 1.5 , 0 ) (1.5 , 0) respectively.

If the shaded area is rotated around the y y -axis to produce an "apple core" structure as shown above, what is the volume of this "apple core"?

Give your answer to the nearest 3 decimal places.


The answer is 2.943.

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1 solution

In order to find the volume of revolution, let us divide the "apple core" into parts by the blue lines y = 3 2 y = \dfrac{\sqrt{3}}{2} and y = 3 2 y = \dfrac{-\sqrt{3}}{2} .

The volume generated between the blue lines (called V 1 V_{1} ) is bounded by the circle graph on the right: ( x 1.5 ) 2 + y 2 = 1 (x - 1.5)^2 + y^2 = 1 .

Then x 1.5 = 1 y 2 x - 1.5 = -\sqrt{1 - y^2} (Note that ( 0.5 , 0 ) (0.5 , 0) is on the graph, so the sign must be minus.)

Hence, x 2 = 13 4 3 1 y 2 y 2 x^2 = \dfrac{13}{4} - 3\sqrt{1 - y^2} - y^2 .

Then V 1 = 3 2 3 2 π ( x 2 ) d y V_{1} = \int_{\frac{-\sqrt{3}}{2}}^{\frac{\sqrt{3}}{2}} \pi(x^2) \ \mathrm{d}y

= 2 0 3 2 π ( 13 4 3 1 y 2 y 2 ) d y 2 \int_{0}^{\frac{\sqrt{3}}{2}} \pi( \dfrac{13}{4} - 3\sqrt{1 - y^2} - y^2) \ \mathrm{d}y

= 2 π [ 13 4 y 3 2 ( a r c s i n ( y ) + y 1 y 2 ) y 3 3 0 3 2 ] 2\pi[\dfrac{13}{4}y - \dfrac{3}{2}(arcsin(y) + y\sqrt{1 - y^2}) - \frac{y^3}{3}|_{0}^{\frac{\sqrt{3}}{2}}]

= 2 π ( 9 8 ( 3 ) π 2 ) 2\pi(\dfrac{9}{8}(\sqrt{3}) - \dfrac{\pi}{2})

Now the volume above the blue line is created from the area bounded between the plus & minus signs of the graph: ( x 0.5 ) 2 + y 2 = 1 (x - 0.5)^2 + y^2 = 1 .

Thus, the rest of the volume = 2 π 3 2 1 ( 5 4 + 1 y 2 y 2 ) ( 5 4 3 1 y 2 y 2 ) d y 2\pi\int_{\frac{\sqrt{3}}{2}}^{1} {(\dfrac{5}{4} + \sqrt{1 - y^2} - y^2) - (\dfrac{5}{4} - 3\sqrt{1 - y^2} - y^2)} \ \mathrm{d}y

= 2 π 3 2 1 2 1 y 2 d y 2\pi\int_{\frac{\sqrt{3}}{2}}^{1} 2\sqrt{1 - y^2} \ \mathrm{d}y

= 2 π [ a r c s i n ( y ) + y 1 y 2 3 2 1 ] 2\pi[arcsin(y) + y\sqrt{1 - y^2}|_{\frac{\sqrt{3}}{2}}^{1}]

= 2 π [ π 2 ( π 3 + 3 4 ) ] 2\pi[\dfrac{\pi}{2} - (\dfrac{\pi}{3} + \dfrac{\sqrt{3}}{4})]

Adding the parts together, the total volume = 2 π [ 7 3 8 π 3 ] 2.943 2\pi[\dfrac{7\sqrt{3}}{8}- \dfrac{\pi}{3} ] \approx 2.943

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