Alaska Science Camps, Fairs & Experiments

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Seals & Beaver,
Floating & Sinking

Coastal people shoot seals in the open ocean. If the water is undiluted by fresh river water, and the seal is fat, the seal will float. If the seal is shot in the river, it will sink. They tend to float more in the winter than in the summer because of fat content. Why is this so?

Upriver people used to shoot beaver in the fall and spring. Grandma Charlie of Sleetmute told me, "In the spring, if the leaves are as big as the beaver's ear, the beaver will sink. Before that, they float." This was important because years ago as we used to shooting beaver in the spring. There is no use to shoot them if they sink and drift away.

A moose or caribou shot in the water will float. A black bear or brown bear shot in the water will sink.

What is happening? Why do some animals float and some sink?

To understand this, we only need to understand specific gravity

Defining and Determining Specific Gravity

What is density? Density explains the relationship of:

• How much something weighs compared to:
• How much space it takes up.
• How dense it is.

An object that has great weight and takes up little space has high density.

An object that has little weight, but takes up much space has low density. Two objects might take up the same space but have different weights. The heavier one has a greater specific gravity.

Water is one of the most common and most important substances in the world, so everything is compared to water. One cubic centimeter of water weighs one gram. Anything that has a volume of one cubic centimeter and weighs one gram is said to have a specific gravity of one.

Anything that has a volume of one cubic centimeter and weighs more than one gram has a specific gravity of more than one. Gold has a specific gravity of over 19. That is, a cubic centimeter of gold will weigh over 19 grams.

With a specific gravity over one, the object will sink in fresh water.

Anything that has a volume of one cubic centimeter and weighs less than one gram is said to have a specific gravity of less than one. Most types of wood have a specific gravity of less than one. They float.

Animals

What determines whether a beaver floats or not? If the beaver's specific gravity is greater than one, it will sink. If it is less than one, it will float. It is that simple.

Let's say that another way. A beaver's body displaces a certain amount of water. If the beaver weighs more than that amount of water, the beaver will sink. Another beaver's body displaces a certain amount of water. If the beaver weighs less than that amount of water, it will float.

When I am swimming, my legs have a specific gravity of less than one. They sink. My body, particularly my chest area, has a specific gravity of less than one, and I therefore float with my back out of the water and my legs hanging downward. My average specific gravity is less than one, so I float.

Again

The specific gravity of fresh water is one.

If an object has a specific gravity of greater than one, it will sink in fresh water.

If its specific gravity is less than one, it will float in fresh water.

The Application

Why does the seal sink in fresh water? The answer is easy. Its specific gravity is greater than one. It is heavier than the water it displaces.

Why then does the seal float in salt water?

The salt content in the ocean water makes a cubic centimeter of salt water heavier than a cubic centimeter of fresh water. Its specific gravity is greater than fresh water.

A seal has a greater specific gravity than fresh water, so it sinks. However, the specific gravity of a seal is less than the specific gravity of salt water, so the seal floats.

This science principle works constantly. Fresh water will actually float on salt water because it is lighter. It will float until they mix. At the mouth of Alaska's rivers, the water on top is less salty than that on the bottom.

Warm water will float on colder water because its specific gravity is less. Water in any condition will float on mercury because the specific gravity of mercury is over 13. A copper penny will sink in water, but float on mercury.

Back to the moose, caribou, black bears, and brown bears. The bears sink because their specific gravity is much greater than one. Moose and caribou float, partly because their hair is hollow, but also because their bodies aren't as dense as the bears. The average if their specific gravity is less than one. Their horns and bones tend to sink, but their hair and lungs float.

A Story

I heard a funny story 30 years ago. A bear was swimming across the river in front of a village. Four men hopped into a long riverboat. Knowing that the bear would sink if they shot it, they put a rope around its neck, planning to drown it behind the boat. The pilot accelerated the boat to pull the bear under the water. However, the rope they used was tied to the front of the boat. It was shorter than the long boat. The bear came alongside the boat with the rope around its neck, and crawled into the boat. With two men on either side of the bear, no one dared to shoot. The pilot crashed into the bank as everyone dove out of the boat. The bear, still dripping, with the rope around its neck, followed the two men out of the front of the boat. On the beach it met its end. Unfortunately for the four hunters, the event took place with the whole village watching.

They knew about the bear's specific gravity, but didn't take time to estimate the rope's length. They got A in science but a D-minus in math that day!

Back to the Question

We could ask, "Why do the beaver sink when the leaves are as big as his ear, but not before?" The answer is: the beaver loses some of his fat after breakup when the leaves are growing. Fat has a very low specific gravity and helps keep the beaver afloat.

If you shoot a beaver and it sinks, it will stay on the bottom. If there isn't much current, you can see little bubbles coming up, and snag the beaver with a hook on a long pole. Oldtimers used to split the end of a long willow. They poked the bottom until they found the beaver. They then pushed and twisted the willow until the beaver's hair was caught in the end of the willow. They slowly drew the beaver to the surface.

If you lose the beaver, it will float in a couple of days, as the gasses produced by decomposition will increase the size of the beaver, and therefore decrease the specific gravity to less than one. Of course, at that time, it would not be fit to eat.

A seal that sinks in fresh water will also float in a few days.

Other applications

1. Gold mines usually separate gold from the other rocks by a two step process. First they screen and size the material. Then they use the very high specific gravity of gold to separate it from the other rocks. Water and the ore are kept in motion down the sluice box. Gold and black sand will always settle to the bottom before the other materials because of their very high specific gravity.

"Country rock" that accompanies gold has a specific gravity of 2.5 to 3.5. It quickly goes to the top while gold and other heavy metals go to the bottom of the sluice box, jig, or recovery system. The country rock is washed away and the gold and black sand remain.
2. A battery tester for a 12 volt automotive battery tests the specific gravity of the acid in the battery. The acid in a charged battery has a greater specific gravity than the acid of a discharged battery. The float in the tester will float higher in the acid of a charged battery than a discharged battery.
3. The tester for antifreeze in a car or truck works the same as the battery tester. It doesn't test at what temperature the antifreeze becomes solid. It only measures the specific gravity of the antifreeze.

From a built-in chart based on the manufacturer's experiments, the tester indicates the freezing point of the fluid.

 EXPERIMENTS, PROJECTS AND QUESTIONS

• Test eight to ten different small objects for their ability to float in fresh water. Put a mark on the waterline of the objects that float. Put as much salt or sugar into the water as will dissolve. Test each of the objects again. Do they float higher or lower? Do some that sank previously now float?
• Try floating the same objects in other liquids. Do they float higher or lower? (Liquid laundry detergent, pancake syrup, shampoo, rubbing alcohol, etc.)
• Does stove oil float on water, or water on stove oil? First predict what will happen, then test the above eight to ten objects in stove oil. Which float and which sink?
• Weigh a liter of fresh water. In the same container, weigh an identical amount of ocean water. Can you determine the difference on your scale or balance, or is the difference too small to be detected?
• Put food coloring into hot water. Can you pour the colored hot water into a container of cold water gently enough to see the hot water float on the cold water? Do this again with colored, salted hot water. Is there a difference?
• If there is a lake free of ice, take the temperature of the water at the surface and again at the bottom. Is there a difference? How could you explain this in terms of specific gravity?
• In the winter, it is warmer at higher elevations than on the rivers (if there is no wind.) Why is this? Why do you think moose migrate from the rivers to the mountains in October until the snow gets too deep in January and February?
• On a very cold day, take the temperature on the river. Go quickly to a high point and record the temperature. What is the difference? How could you explain this regarding specific gravity of colder and warmer air?
• If your class or family goes on a trip to a location with a swimming pool, test each student or family member to see who can float the highest, and who sinks the deepest. What conclusions can you draw about flotation and body types?
• Do you think someone would float higher or sink deeper in the Great Salt Lake in Utah than they do in the ocean? Why?
• Ask the oldtimers in your area which animals float and which ones sink and if that changes with seasons. If you live where a river flows into the ocean, ask them about the animals in both fresh and salt water. How do people catch seals in fresh water? How do they keep from losing them?
• Test different animal's fur for flotation. Which ones float and which ones sink?
 Finding and Developing a Science Fair Project Examples of "Observe and Think" Projects 200+ Ideas for Science Fairs! Traditional Lighting Firestarting Traditional Firemaking Sharpening Fishing with Lures Rabbit Snares Spearing Fish and the Refraction of Light Chill of the Campfire Solution vs Suspension Seals & Beaver, Floating & Sinking Steaming Selecting a Birch Tree Spruce & Other Roots Spruce Gum Spear Throwing Berry Pickers Drum Frames Conclusion

A partner with the Alaska Rural Systemic Initiative

This material is based upon work supported by the National Science Foundation under Grant No. 0086194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Contents

Camps as an Environment for Science & Culture

Culturally Relevant Science Fairs

Experiments

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