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A large wooden plate of area \small 10 \mathrm{~m}^2 floating on the surface of a river is made to move horizontally with a speed of  \small 2 \mathrm{~m} / \mathrm{s} by applying a tangential force. River is \small 1 \mathrm{~m} deep and the water in contact with the bed is stationary. Then choose correct statement-

Option: 1

\text { Velocity gradient is } 2 \mathrm{~s}^{-1}


Option: 2

\text { Velocity gradient is } 1s^{-1}


Option: 3

Force required to keep the plate moving with constant speed is 0.02 \mathrm{~N}


Option: 4

Force required to keep the plate moving with constant sped is 0.01 \mathrm{~N}


Answers (1)

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\begin{gathered} \text { Velocity gradient }=\frac{\Delta V}{\Delta h}=\frac{2 \mathrm{~m} / \mathrm{s}}{1 \mathrm{~m}}=2 \mathrm{~s}^{-1} \\ F=\eta A \frac{\Delta V}{\Delta h}=10^{-3} \times 10 \times 2 \\ F=0.02 \mathrm{~N} \end{gathered}

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