Do you wait for ketchup?
Learning Objectives
After completing this lesson, the student will be able to describe similarities and differences in loads on a body based on fluid properties.
Standards
- NGSS HS-ETS1-3
- CCSS.Math.Practice.MP1
- CCSS.Math.Practice.MP2
- CCSS.Math.Practice.MP4
Supplies
- A collection of different types of fluids (water, ketchup, honey, etc…)
- Lid from water tub, or similar light weight, non-porous surface
Viscosity
Viscosity is a measure of the stickiness of a fluid. A fluid that flows easily has a lower viscosity. A fluid that flows slowly has higher viscosity. Which do you think has a higher viscosity: water or honey?
Experiment: gather a few kitchen liquids (work with a teacher or parent to verify you are using safe items – e.g. water, honey, ketchup, vegetable oil, molasses) and put a drop of each on a non-porous surface. Tilt the surface so the liquids flow downhill. Whichever one moves fastest has the lowest viscosity. Rank your liquids in order of lowest to highest viscosity.
Viscosity is relevant for our blimps because it is the cause of a boundary layer of fluid around the blimp. When a body is in a fluid flow, viscosity causes what’s known as a no-slip condition – that the fluid at the body has zero speed relative to the body. As you move further from the body, the fluid speed will equal the speed of the free stream flow. This results in a drag force on the body. If you are interested in diving into this more, a great lesson on this is available through MIT’s Open Courseware: https://ocw.mit.edu/courses/mechanical-engineering/2-016-hydrodynamics-13-012-fall-2005/readings/2005reading5.pdf .
Non-Newtonian Fluids
A particularly interesting, and fun, type of fluid is known as a non-Newtonian fluid. In Newtonian fluids, the viscosity does not change based on how you push on it. A non-Newtonian fluid’s viscosity does change with how you push on it. Quicksand is an example of a non-Newtonian fluid, because the harder something stuck in quicksand struggles, the faster they sink. A common experiment demonstrating a non-Newtonian fluid is to make a mixture of corn starch and water. If you press slowly on the corn starch and water mixture, your hand will sink in like it is a liquid. But if you press quickly on it, then it can feel almost like a solid surface. Check out this video: https://www.youtube.com/watch?v=v581Y50-bow, or do the experiment yourself!
Ketchup is a non-Newtonian substance. Go ahead, play with your food and see if you can determine the easiest way to make ketchup flow out of a bottle!
Next Steps
This lesson was experimental. If you want to continue with the hands-on approach, tie together what you’ve learned about fluids and structures with this lesson.
If you’d like to step up your mathematical analysis skills applied to fluid dynamics, move on to our advanced level lesson on added mass and drag.
Last updated: November 23, 2022.
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