Key concepts
Physics
Friction
Solids
Liquids

Introduction
Have you ever tried to squeeze honey or syrup out of a bottle at breakfast on a chilly winter morning? Do you notice that it's harder to do that than on a hot summer day? As the liquid gets colder, its viscosity, or resistance to flow, increases. Viscosity is a properly of liquids that can be very important in very different applications—from how the syrup flows out of your bottle to how blood flows through the human body to how lava flows out of a volcano. In this project you will learn a little bit about viscosity by holding a marble race!

Background
You experience friction all around you. It is what allows your shoes to grip the floor so you don't slip and it's what makes your bike come to a stop when you squeeze the brakes. This type of friction is a force that resists motion between two solid objects. Liquids, however, have friction, too—not just against solids (for example, water against a drinking glass)—but also internal friction, the liquid against itself. This internal friction is called viscosity. Different liquids have different viscosities, which means some liquids flow more easily than others. You will notice this if you think about squirting water out of a bottle or squirt gun. Imagine how much harder that would be to do with cold syrup!

There are several different ways scientists can measure the viscosity of a liquid. One method is called a "falling sphere viscometer," in which you drop a sphere (such as a marble) through a tube filled with liquid. By measuring how long it takes the marble to fall and how far it travels, you can figure out the liquid’s viscosity. You won't need to do any calculations in this activity—but you will get to "race" marbles by dropping them in different liquids. Will viscosity affect how fast the marbles fall? Try this project to find out!

Materials

• About a dozen equal-size marbles
• At least two equal-size tall, transparent drinking glasses (the taller the better)
• Assorted liquids from around your kitchen you have permission to use, such as water, syrup, honey, molasses, olive oil, vegetable oil, etcetera
• Strainer or colander
• A flat surface that can have liquids (water, oil, etcetera) spilled on it—or protection (such as a large trash bag) for the surface
• Optional: Extra bowls/containers and/or a funnel (for storing and reusing the liquids you use for the activity, if you do not want to throw them away)
• Optional: Volunteer to help you see which marble hits the bottom first

Preparation

• If you want to save and reuse the liquids you use from the activity, make sure you thoroughly wash your marbles and drinking glasses with soap and water, then dry them completely. This will assure they are clean and you do not get your liquids dirty.
• Prepare a work space on your flat surface and ensure that it is ready for any accidental spills (of water, oil, etcetera).

Procedure

• Fill your two (or more) drinking glasses with each of your different liquids to the same height. (To avoid spilling when you drop the marbles in do not fill them all the way to the brim.)
• Which liquid do you think has a higher viscosity? Can you tell when you pour them into the glasses? Do you think the marble will fall faster through one of the liquids?
• Hold one marble in each hand, just above the surface of the liquid in each glass.
• Watch the glasses closely. Be prepared to watch the bottom to see which marble hits first. If you have a volunteer, have them look at the glasses, too.
• Let the marbles go at exactly the same time.
• Observe which marble hits the bottom of the glass first.
• Which marble won the "race"? Do your results match your prediction?
• Repeat the activity with a few more marbles to see if you get the same results. (Use clean, dry marbles each time.)
• If you have more than two different liquids, you can try racing marbles in other liquids to see what happens.
• Through which liquid do the marbles fall the fastest? The slowest?
• Extra: What happens if you drop different types of marbles (for example, steel marbles versus glass marbles) or different size marbles? Do the results of your races change?
• Extra: What happens if you change the temperature of a liquid? Have an adult help you cool some syrup in the refrigerator and heat some on the stove or in the microwave. What happens if you do a race with cold versus warm syrup instead of room-temperature syrup? How does temperature affect the liquid’s viscosity? Is the temperature effect stronger on some liquids than it is for others?

Observations and results
When pouring your liquids, you might have noticed that some of them were "thicker" or harder to pour. These are the more viscous liquids. You can also think about what these liquids are like when you use them everyday. For example, what would happen if you poured water on pancakes? Would it flow slowly like syrup or spread out very quickly? What about if you tried to pour and drink a glass of syrup? Taste (and healthfulness) aside, would that be harder than drinking a glass of water?

You should have observed that the marbles fell more slowly through more viscous liquids (such as syrup) than through less viscous liquids (such as water). This is because the more viscous liquids have more resistance to flow, making it more difficult for the marble to travel through them. It might be hard to tell the difference between the results for some liquids, however—especially if your glasses are not very tall. This is why it is important to do multiple trials and have a volunteer help watch the marbles.

Cleanup
If you want to keep the remaining liquids for future use, have an adult help you pour them back into storage containers. (Use the strainer to remove the marbles). Otherwise, have an adult help you dispose of the liquids properly. Be careful because pouring some viscous liquids (such as cooking oil) down the sink can clog the drain.

More to explore
Race Your Marbles to Discover a Liquid's Viscosity, from Science Buddies
What Is Viscosity?, from Princeton University
It's a Solid… It's a Liquid… It's Oobleck!, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies