Water flowing from a sphere

Calculus Level 5

A hollow sphere of mass m m and radius 1 1 metre is placed with it's centre at origin in x y z xyz -space. It is filled completely with water of mass m m . So, now the total mass of system is 2 m 2m .

A small hole is made at t = 0 t=0 at the bottom and water starts flowing out of the sphere with a constant volume flow rate.

If the maximum distance of centre of mass of the system from origin during the time t = 0 t=0 to the time water completely flows out from the sphere is a 1 n b 1 n {a}^{\frac{1}{n}}-{b}^{\frac{1}{n}} meters, then what is the value of a + b + n ? a+b+n?

Details and assumptions

  • a , b , n a,b,n are distinct integers such that 0 < a , b , n < 20 0<a,b,n<20 .

  • While calculating centre of mass of the system don't consider the water that flown out from the sphere. Only consider the water that is present inside the sphere.


The answer is 25.

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1 solution

Invert the universe so that the water is now flowing out through top. Now, at a certain instant, the surface of water(blue) is at distance x x from the center.

At this instant, mass of water body is given by:

m w = x 1 ρ π ( 1 t 2 ) d t mw=\int _{ x }^{ 1 }{ \rho \pi (1-{ t }^{ 2 })dt }

where ρ \rho is the density of liquid.

therefore, center of mass of water body is given by

x 1 = x 1 ρ π t ( 1 t 2 ) d t m w x_{1}=\frac { \int _{ x }^{ 1 }{ \rho \pi t(1-{ t }^{ 2 })dt } }{ mw}

this means that center of mass of whole system is:

K = x 1 ρ π t ( 1 t 2 ) d t x 1 ρ π ( 1 t 2 ) d t + m K=\frac { \int _{ x }^{ 1 }{ \rho \pi t(1-{ t }^{ 2 })dt } }{ \int _{ x }^{ 1 }{ \rho \pi (1-{ t }^{ 2 })dt } +m }

where m m is mass of container.

now, put f ( t ) = ρ π ( 1 t 2 ) f\left( t \right) =\rho \pi (1-{ t }^{ 2 }) , then the equation reduces to:

K = x 1 t f ( t ) d t x 1 f ( t ) d t + m K=\frac { \int _{ x }^{ 1 }{ tf\left( t \right) dt } }{ \int _{ x }^{ 1 }{ f\left( t \right)dt } +m }

now putting the condition d K d x = 0 \frac { dK }{ dx } =0 , we get that:

x = x 1 t f ( t ) d t x 1 f ( t ) d t + m x=\frac { \int _{ x }^{ 1 }{ tf\left( t \right) dt } }{ \int _{ x }^{ 1 }{ f\left( t \right)dt } +m }

x = K \Rightarrow x=K (when K K is maximum)

Now, integrating and cross multiplying, we get:

x 4 6 x 2 + 24 x 3 = 0 x^4-6x^2+24x-3=0

solving this equation yields the required value of x x , which is 12 1 4 9 1 4 m e t r e \boxed{{12}^{\frac{1}{4}}-{9}^{\frac{1}{4}} metre}

thus, answer is: 12 + 9 + 4 = 25 12+9+4=\boxed{25}

How did you solve the fourth degree polynomial?

Jacob Swenberg - 5 years ago

How can volume flow rate remain constant with height of water changing?Is area of crossection of the hole also changing? @Ronak Agarwal

Adarsh Kumar - 5 years, 11 months ago

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It's a maths problem, hence I have not considered the fact that how the flow is kept constant.

Ronak Agarwal - 5 years, 11 months ago

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Ok,fine. Thank you for replying.

Adarsh Kumar - 5 years, 11 months ago

Wishing you a wonderful time at whichever IIT you choose to join,bhaiya!

Adarsh Kumar - 5 years, 11 months ago

So far it is the best cm question I've on brilliant. Even I did in the same way(the iitian way) :P

Aditya Kumar - 5 years, 11 months ago

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