Clockwork Orange Juice

We all love to have chilled drinks in summers. So here's a nice problem related to orange juice.

A jug contains 15 glasses of Orange Juice. When you open the tap at the bottom it takes 12 seconds to fill a glass with juice. If you leave the tap open, how long will it take to fill the remaining 14 glasses and thus empty the jug. This time can be expressed as a b c d \frac { a\sqrt { b } }{ \sqrt { c } -\sqrt { d } } , then find a + b + c + d a+b+c+d .

Details and Assumptions :

  • Consider the jug to be a cylinder.

  • all 15 glasses are indistinguishable.

  • b , c b, c and d d are square free positive integers.

Try Fluid Mechanics-1 and Fluid Mechanics-3


The answer is 55.

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3 solutions

Assume that,

  • the amount of liquid in one glass is W W ,
  • the area of the base of the jug is A A ,
  • the area of the opening at the bottom of the jug is B B , and
  • the height of the jug is h h .

Initially, the jug contains 15 W 15W amount of liquid.

Because the jug is cylindrical, then the volume of liquid inside is given by,

V = A h V = Ah

h = V A h = \frac{V}{A} (1)

By Toricelli's Law, the velocity of the liquid pouring out of the opening is given by,

v = 2 g h v = \sqrt{2gh}

Thus,

d V d t = B 2 g h \frac{dV}{dt} = -B \sqrt{2gh} using eq (1)

d V d t = B 2 g V A \frac{dV}{dt} = -B \sqrt{\frac{2gV}{A}}

V 1 / 2 d V = B 2 g A d t V^{-1/2} dV = -B\sqrt{\frac{2g}{A}} dt (2)

Since it takes 12 s e c o n d s 12 seconds to fill one glass of juice, we could integrate eq (2),

15 W 14 W V 1 / 2 d V = 0 12 B 2 g A d t \int_{15W}^{14W}{V^{-1/2} dV} = \int_{0}^{12}{-B\sqrt{\frac{2g}{A}} dt}

2 ( 14 W 15 W ) = 12 B 2 g A 2(\sqrt{14W} - \sqrt{15W}) = -12B\sqrt{\frac{2g}{A}}

15 14 6 = B 2 g A W \frac{\sqrt{15}-\sqrt{14}}{6} = B \sqrt{\frac{2g}{AW}} (3)

Let T T be the time it takes to fill the remaining glasses, we would integrate eq (2) again,

14 W 0 V 1 / 2 d V = 0 T B 2 g A d t \int_{14W}^{0}{V^{-1/2} dV} = \int_{0}^{T}{-B\sqrt{\frac{2g}{A}} dt}

2 14 W = B T 2 g A -2\sqrt{14W} = -BT\sqrt{\frac{2g}{A}}

2 14 = T ( B 2 g A W ) 2\sqrt{14} = T(B\sqrt{\frac{2g}{AW}}) using eq (3)

2 14 = T ( 15 14 6 ) 2\sqrt{14} = T(\frac{\sqrt{15} - \sqrt{14}}{6})

12 14 15 14 = T \frac{12\sqrt{14}}{\sqrt{15} - \sqrt{14}} = T

Thus,

( a , b , c , d ) = ( 12 , 14 , 15 , 14 ) (a, b, c, d) = (12, 14, 15, 14)

a + b + c + d = 12 + 14 + 15 + 14 = 55 a + b + c + d = 12 + 14 + 15 + 14 = \boxed{55}

Rahul Chandani
Apr 2, 2015

I used calculus method for finding it.. It took jst some time to think upon it :)

Priyesh Pandey
Apr 2, 2015

Use eqn of continuity and toricelli's eqn/

Was at right path ...

Pawan pal - 5 years, 1 month ago

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