Focus question

How does the viscosity of a liquid affect the motion of an object moving in the liquid?

Essential materials

• Water
• Baby oil (or some other easily obtainable light oil)
• Clear or light colored shampoo
• Clear plastic bottle about 444 mL (15 oz) capacity or slightly larger, with tightly fitting cap
• 35 mm film canister or similar small object
• Stopwatch that measures to 0.1 or 0.01 seconds
• Glass marble small enough to fit through the mouth of the bottle
• Permanent marking pen
• Safety goggles
• Chart like the one below to record your results

Main ideas and background information

• Viscosity is the property of a fluid that causes it to resist flowing. Thick liquids such as oil and shampoo are more viscous than water.
• A viscous liquid resists the motion of an object moving in it. The more viscous a liquid is, the more it will resist the movement of an object. Thus, an object can move more freely in a liquid of low viscosity, such as water, but moves more slowly in a liquid of higher viscosity, such as oil or shampoo.

Procedural tips

• Be sure to have students draw the lines all the way around the bottle. They will need to have their eyes level with each line when making their time measurements, and this will help them to do so. Note how the camera is positioned in the photographs for step #7 for the top and the bottom line.
• When students attempt to measure the time it takes in step #4 for the marble to drop in water, help them realize that they are measuring what is essentially their reaction time. The marble drops so quickly in such a non viscous liquid that it is not possible to measure the time by hand with a stopwatch.
• For the ramp in step #5, place the bottle cap on a small object to make a ramp with an angle of no more than about 10 degrees or so. This will be enough to ensure a reasonably slow rolling speed for the marble. (Note: Galileo used a similar technique to “dilute” the acceleration due to gravity. The description of his ramps is found on pages 178 and 179 of his book, Two New Sciences.)
• For the most consistent results, students should make sure that the marble is in the same starting position in step #6 before lowering the end of the bottle in step #7. If the starting position is a centimeter or two above the first line, it will help the timer start the stopwatch more consistently as the marble reaches the first line. 
• In step #10, allow as much water as possible to drain from the bottle before adding the marble and oil.
• In optional step #11, the contrast with the more viscous shampoo will be dramatic, especially if students replicate the ramp measurement.

Discussion

1. How was the time it took for the marble to roll down the ramp in water different in oil? (It took a longer time for the marble to roll down in oil than it did in water.)
2. What is your explanation for this difference? (The oil is more viscous than the water. The water does not hold the marble back as much.) 
3. Looking at your data, what do you think might happen if the angle of the ramp had been steeper? Less steep? (For a steeper angle, the marble would roll faster and the rolling times would be shorter. For a less steep angle, the times would longer.)
4. How might this activity be useful in your everyday life? (For example, students might notice a difference in the force required to stir various liquids.)

Assessment

Are students able to describe how the viscosity of a liquid affects the motion of an object moving in the liquid? (A liquid with a low viscosity, such as water, does not resist the motion of an object moving in it very much. A liquid with a higher viscosity, such as oil or shampoo, resists the motion of an object moving in it to a greater extent.)

Extensions and further investigations

• Have students repeat the experiment using
  • other liquids.
  • a marble of a different size.
  • a bottle of a different size (with a different rolling distance).
  • different ramp angles.
• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

• Developing and manufacturing products such as
  
  • mineral oils and hydrocarbons,
    
  • cosmetics and pharmaceuticals
    
  • foods (such as honey, beer, gelatin, etc.)
    
  • chemicals (such as polymers, solvents, resins, adhesives)
    
  • paints, varnishes, inks, lacquers
    
  • detergents
    
  • fuel (diesel oil, gas/petrol, paraffin)
    
  • automotive fluids, coolants
    
• Petroleum engineers developing products with different viscosities
• Paper industry (additives, pigments, emulsions)
• Maple syrup production

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B - Physical Science: B1, B2

Grades 9-12
Standard B – Physical Science: B2

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS2: Sc3, 1a; Sc4, 2b, c; BoS, 1a, c, d, 2a
KS3: Sc3, 1b; Sc4, 2a, b, c, d; BoS, 1a, d, e
KS4: Sc3, 2a; Sc4, 2a, e, h, l; BoS, 1a, d, e

Glossary/vocabulary

viscous
viscosity

Resource links

Hawai'i Space Grant College, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i
A good general treatment of viscosity, with some additional information about the viscosity of volcanic lava.

Lab Dad's Laboratory
This site has instructions for a slightly different approach to experimenting with viscosity.

Discovery Center Museum
Here are some ideas for a similar experiment using air bubbles instead of marbles.

…more Science Lab projects about…
Air & Space
Earth Science
Electricity & Magnetism
Properties of Liquids

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