EasyPhysicsHigh School (11–12)Mechanics › Fluid Flow, Bernoulli's Principle and ViscosityConceptualBoard Exam
Answer, and give a reason. Why does a river flow fastest where it is narrowest? …
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Answer, and give a reason.
- Why does a river flow fastest where it is narrowest?
- Why do blood vessels that narrow make the heart work harder?
- Why does putting your thumb over the end of a hose make the water go farther?
Need a reminder? The key ideaShow
Continuity: for steady flow of a liquid, ; the volume flow rate is the same through every section. The area goes as the diameter squared.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The same volume must pass each section every second, so where the area is small the speed is large.
Step 2. (b) Blood must speed up through the narrow part, and to push it through, the heart must raise the pressure behind it.
Step 3. (c) The thumb makes the opening smaller, so the water leaves faster and travels farther before it lands.
Answer: Continuity, , in each case.
Watch out. The thumb does not add water; the flow rate stays about the same.
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