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Mechanical Properties Of Fluids

Question
CBSEENPH11020053

Derive the expression for viscous force acting on spherical body of radius r moving with velocity v through viscous liquid of coefficient of viscosity η.

Solution
Let space straight F space proportional to space straight r to the power of straight a space

space space space space space space space space space space proportional to space straight v to the power of straight b

space space space space space space space space space space proportional to space straight eta to the power of straight c space

space space space space space straight F space proportional to space straight r to the power of straight a space straight v to the power of straight b space straight eta to the power of straight c space

straight i. straight e. comma space straight F space equals space straight k space straight r to the power of straight a space straight v to the power of straight b straight eta to the power of straight c space space space space space space space space space space space... space left parenthesis 1 right parenthesis thin space

straight k space is space the space constant space of space proportionality. space

Now comma space putting space the space dimension space formula space of space
each space quantity space in space equation space left parenthesis 1 right parenthesis comma space we space get space

space space space space space space space space space space left square bracket straight M to the power of 1 straight L to the power of 1 space straight T to the power of negative 2 end exponent right square bracket space equals space left square bracket straight M subscript straight o straight L to the power of 1 straight T subscript straight o right square bracket to the power of straight a space left square bracket straight M to the power of 0 straight L to the power of 1 straight T to the power of negative 1 end exponent right square bracket to the power of straight b space left square bracket straight M to the power of 1 straight L to the power of negative 1 end exponent straight T to the power of negative 1 end exponent right square bracket to the power of straight c space

rightwards double arrow space space space left square bracket straight M to the power of 1 straight L to the power of 1 space straight T to the power of negative 2 end exponent right square bracket space equals space left square bracket straight M to the power of straight c space straight L to the power of straight a plus straight b minus straight c end exponent space straight T space to the power of negative straight b minus straight c end exponent right square bracket space

Equating space the space powers space of space straight M comma space straight L space and space straight T space on space both space sides comma space we space have space

straight c space equals space 1

straight a plus straight b minus straight c space equals space 1

minus straight b minus straight c space equals space minus 2 space

On space solving comma space we space get

straight a space equals space 1 comma space straight b space equals space 1 comma space straight c space equals space 1 space

Putting space the space values space of space straight a comma space straight b space and space straight c space in space equation space left parenthesis 1 right parenthesis comma space we space get

straight F space equals space straight k space straight r space straight v space straight eta