EasyPhysicsHigh School (11–12)Mechanics › Fluid Flow, Bernoulli's Principle and ViscosityConceptualBoard Exam
Answer, and give a reason. Why do big hailstones hit the ground much harder than small ones? …
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 do big hailstones hit the ground much harder than small ones?
- Why is honey warmed before it is poured?
- Why does dust stay in the air of a room for hours?
Need a reminder? The key ideaShow
Viscosity: the force to slide a plate of area at speed over a film of thickness is . Stokes' law: a small sphere feels , so it falls at a terminal speed .
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) Terminal speed grows as (until the air flow turns turbulent), so big stones fall far faster, and they are heavier too.
Step 2. (b) Warming lowers a liquid's viscosity, so the honey flows quickly.
Step 3. (c) Dust grains are so small that their terminal speed is tiny, and gentle air currents keep them up.
Answer: (a) . (b) Lower viscosity. (c) Tiny terminal speed.
Watch out. For gases, warming raises the viscosity.
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