Question

Water is flowing in a horizontal pipe of uniform cross-section. At some place the pipe becomes narrow, then the pressure of water at this place

• A. Remains same
• B. May increase or decrease
• C. Increases
• D. Decreases

Answer 1

The correct option is D Decreases

Applying Bernoulli's equation.
$P+\frac{1}{2}\rho v^2+\rho gh=$ constant
Since the pipe is horizontal,


$P_A+\frac{1}{2}\rho v_A^2+\rho g{h}_A=P_B+\frac{1}{2}\rho V_B^2+\rho g_B$
$[\because h_A=h_B]$
$P_A+\frac{1}{2}\rho V_A^2=P_B+\frac{1}{2}\rho V_B^2.....\left(1\right)$
Now applying the equation of continuity,
$A_A{V}_A=A_B{V}_B$
But $A_B<A_A\Rightarrow V_B>V_A$
Then from equation $\left(1\right)$
$\Rightarrow P_A>P_B$
So, option (D) is correct.