Assertion (A) : When water flows in a uniform horizontal pipe there is a fall in pressure of water along the pipe. Reason (R) : Force is required to maintain the motion of the liquid against friction.

Assertion (A) is true because when a real fluid like water flows through a uniform horizontal pipe, there is a measurable drop in pressure along the direction of flow. According to the Bernoulli equation for ideal fluids, pressure should remain constant in a horizontal pipe of uniform diameter where velocity is constant [t5][t6]. However, real fluids possess viscosity, which leads to internal friction and shear stress at the pipe walls [t1]. Reason (R) correctly explains this phenomenon: to maintain a steady flow, a driving force (provided by a pressure gradient) is required to overcome the resisting force of viscous friction [t1][t7]. Without this pressure drop, the fluid would decelerate due to energy dissipation into thermal energy [t3]. Thus, the pressure gradient is the mechanism that balances the frictional losses, making R the correct explanation for A.