Alaska Science Camps, Fairs & Experiments
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offices at 907-474-1902 or email uaf-cxcs@alaska.edu.  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
 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.- 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.
- 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
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- Test eight to ten differ
 ent
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?
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