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PART - 2 CH - 9 Mechanical Properties of Fluids — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 SciencePhysicsPART - 2 CH - 9 Mechanical Properties of Fluids2 Marks
Question
What is stokes' law? Write its application of this formula.
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Answer

Stokes' law : According to this law the radius of the sphere $r$, which is the viscosity $\eta$ in a fluid, moving with velocity $v$, experiences a gravitational force F due to the viscosity of the fluid which can be expressed by $\vec{F}=$ $-6 \pi \eta r \vec{v}$
Application of Stokes' law :
(i) When water vapour condenses on dust particles then their terminal velocity is proportional to the square of the radius. So smaller drops fall with low velocity and bigger drops with high velocity.
(ii) When a soldier jumps down with a parachute, then initially his velocity increases rapidly but after the parachute opens the air exerts more viscous force upwards due to which the soldier falls down with a constant terminal velocity and lands safely.
(iii) To find the charge of electrons using Millikan's oil drop method, to find the terminal velocity of oil drops Stokes formula is used.
(iv) When water vapour present in air gets condensed on dust particles, small drops are formed. The drops acquire terminal velocity easily because of light weight and appear to be floating in space because of low velocity.

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