A steady non-turbulent stream of water comes out of a tap and falls vertically downward. As it does so, the diameter of the stream appears to get smaller. What is the primary reason?
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This phenomenon is explained by the principle of continuity for incompressible fluids. The flow rate, which is the volume of water passing a point per second, must remain constant. The flow rate (Q) equals the cross-sectional area (A) multiplied by the velocity (v): .
As the water falls, gravity accelerates it, increasing its velocity (v). For the flow rate (Q) to stay constant, the cross-sectional area (A) must decrease. A smaller area means a smaller diameter, making the stream appear thinner.
Final Answer: The water is accelerating under gravity and so the stream must get thinner as the flow rate (velocity times cross-sectional area) must be constant.