Water in a sphere..

Classical Mechanics Level pending

A hollow sphere is filled with water weighing as much as the sphere. It then does pure rolling.

The ratio of the K E KE of the system when the water is liquid to that when it is solid is of the form a b \frac{a}{b} where a a and b b are co prime integers.

Find a + b a+b

Assumption \textbf{Assumption}

\bullet The velocity of the system in both the cases is same.


The answer is 43.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Ronak Agarwal
Aug 6, 2014

When water is liquid in the sphere it will have no rotational kinetic energy also m s p h e r e = m W a t e r {m}_{sphere}={m}_{Water} , so we write kinetic energy of the system as :

K . E 1 = 1 2 m s p h e r e v 2 + 1 2 m w a t e r v 2 + 1 2 ( 2 3 m s p h e r e r 2 ω 2 ) = 4 3 m v 2 {K.E}_{1} = \frac { 1 }{ 2 } { m }_{ sphere }{ v }^{ 2 }+\frac { 1 }{ 2 } { m }_{ water }{ v }^{ 2 }+\frac { 1 }{ 2 } (\frac { 2 }{ 3 } { m }_{ sphere }{ r }^{ 2 }{ \omega }^{ 2 })=\frac { 4 }{ 3 } m{ v }^{ 2 }

Note I am using the conditions of pure rolling that is v = ω r v=\omega r

When water is solid it will also have rotational kinetic energy. So now we write it's kinetic energy as :

K . E . 2 = 1 2 m s p h e r e v 2 + 1 2 m w a t e r v 2 + 1 2 ( 2 3 m s p h e r e ω 2 r 2 ) + 1 2 ( 2 5 m W a t e r ω 2 r 2 ) = 23 15 m v 2 {K.E.}_{2}=\frac { 1 }{ 2 } { m }_{ sphere }{ v }^{ 2 }+\frac { 1 }{ 2 } { m }_{ water }{ v }^{ 2 }+\frac { 1 }{ 2 } (\frac { 2 }{ 3 } { m }_{ sphere }{ \omega }^{ 2 }{ r }^{ 2 })+\frac { 1 }{ 2 } (\frac { 2 }{ 5 } { m }_{ Water }{ \omega }^{ 2 }{ r }^{ 2 })\\ =\frac { 23 }{ 15 } m{ v }^{ 2 }

Dividing these two we have :

K . E 1 K . E 2 = 20 23 \boxed{\frac { { K.E }_{ 1 } }{ { K.E }_{ 2 } } =\frac { 20 }{ 23 }}

Pratik you should mention in the question that we have to assume same velocity in both the cases.

Yeah..i'll edit the question.

Pratik Shastri - 6 years, 10 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...