LESS ON
26
rh y do objects float?
When you throw a rock into water, it sinks. It sinks because a rock is
more dense than water. When you throw wood i nto water, it floats. It
floats because wood is less dense than water.
Objects that are more dense than water sink in water.
Objects that are less dense than water, float on water.
Here is another comparison.
The density of water is 1 gram per cubic centimeter.
The density of steel is nearly 8 grams per cubic centimeter. This is eight
times more dense than water.
Steel, then, should sink in water-and it does. Yet a steel ship that
weighs 100 thousand tons or more floats on water.
How can this be explained? Why do steel ships float?
The answer is simple. A ship may be made of steel. But only a small part
of its volume is steel. Because of the shape of the hull, most of the ship's
volume is air. Air is less dense than water.
The density of a ship, then, is not just the density of the metal. It is the
density of the metal, the air in the hull, and all the other things in the
ship. Altogether, the density is less than the density of water. Objects
that are less dense than water float in water. That is why a ship floats.
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UNDERSTANDING FLOATING
A flat piece of metal has no air trapped within its border. All its volume is metal. All its density
is metal.
Figure A
Figure B
Metal is more dense than water. It sinks in water.
What happens when you shape the metal like a bowl-or a ship?
Figure C
Figure D
The new shape has a volume of metal-and a great deal of air.
The new shape has a new density. It is the density of the steel as well as the air. Air has a low
density.
Altogether, the density is less than the density of water. The shaped steel floats on water.
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COMPLETING SENTENCES
Choose the correct word or terra ,for each statement. Write your choice in the spaces provided.
Objects that are more dense than water
in water,
float, sink
2.
Objects that are less dense than water
sink, float
A flat piece of metal
in wat
have air trapped within its sides.
does, does not
4.
If you place a flat piece of metal on water, it will
sink, float
5.
A ship is shaped like a
table top, bowl
6.
A bowl has
more, less
7.
volume than the flat metal it is made of.
Most of the volume of a ship is
water, air, metal
8.
Air is mostly
liquids, gases
9.
10.
Gases are
more, less
dense than metal.
Filling a ship with water would make it
more, less
dense.
MATCHING
Match each term in Column A with its description in Column B. Write the correct letter in the space
provided.
Column A
1. volume
a)
measured in grams per cubic centimeter
2.
b)
measured in Newtons per square centimeter
c)
length x width x height
density
3. pressure
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Column B
4.
floats
d) an object more dense than water
5.
sinks
e) an object less dense than water
CHANGES IN DENSITY
The deeper w e go in water, the more water
there is above us. The mass of the water above
us gets greater and greater. Therefore, the
pressure becomes greater and greater. This
pressure squeezew the water making it more
dense. Some objects are too dense to float on
water's surface. These objects sink down to
deeper water. The objects will continue to sink
until they reach water that is as dense as they
are. There, the objects will float.
Figure E
COMPLETING SENTENCES
Choose the correct word or term for each statement. Write your choice in the spaces provided.
1. The water at the surface of the container is
bottom of the container.
2.
3.
The water at the bottom of the container is
top of the container.
The water is pressing
than the water at the
higher, lower
than the water at the
higher, lower
on the top surface.
upward, downward
4.
The water is pressing
on the bottom surface.
upward, downward
5.
6.
The pressure of the water at the top of the container is
pressure on the bottom of the container.
The pressures ,-
greater, less
than the
equal.
are, are not
7.
The water is
8.
The water is
9.
more, less
dense at the top of the container.
dense at the bottom of the container.
more, less
floa
The object
the top of the container.
does,does not
10.
The object
float at the bottom of the container.
does, do c's n;-)t
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DENSITY AND SEPARATING SUBSTANCES
oil
vinegar
spices
Figure F
Figure G
Figure F shows salad dressing before it is mixed.
1.
The parts are separated because they have different
sizes, densities
2.
Which part is most dense?
3.
Which part is the least dense?
How do you know?
How do you know?
Study the mixture in Figure G. Then answer the questions.
4.
Name the solids in the glass.
5.
Name the liquids.
6.
Which solid is the most dense?
7.
Which solid is the least dense?
8.
Which liquid is the most dense?
9.
Which liquid is the least dense?
10.
Which of these substances have densities greater than water?
11.
Which have densities less than water?
12.
Which substance is the most dense?
13.
Which substance is the least dense?
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SUBMARINES
Figure H
A submarine has special tanks called ballast [BAL-ust] tanks. Ballast tanks are used to make the
submarine heavier or lighter. When the ballast tanks are filled with seawater, the submarine
submerges, or goes under water. When the water is forced out of the ballast tanks, the submarine
rises and surfaces.
1.
Adding ballast water makes a submarine
dense.
more, less
2.
Forcing ballast water out makes a submarine
dense.
more, less
3.
When a submarine is submerging its density is
than water.
greater, less
4.
When a submarine is surfacing its density is
greater, less
5.
than water.
When a submarine is submerging, its ballast tanks are
full, empty
TRUE OR FALSE
In the space provided, write "true" if the sentence is true. Write "false" if the sentence is false.
1.
A bar of steel will float on water.
2.
Then density of steel is greater than water.
3.
A steel ship will float on water.
4.
A steel ship is mostly metal.
5.
The density of a steel ship is greater than the density of water.
6.
Part of every floating object is below the water.
7.
Water pressure increases the deeper you go.
S.
An object will float in water if it is less dense than water.
9.
An object will sink in water if it is less dense than water.
10.
A submarine can change its density.
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WORD SEARCH
The list on the left contains words that tort haazce used in this Lessen. Find and circle each r xord where it
appears in the hex. The spellings ana i go in any direction: uzu, down, left, right, or diagonally.
FLOAT
BALANCE
PRESSURE
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REACHING OUT
Gases separate by density. So do liquids. Even
a solid, when it is in a gas or a liquid, will find
its level by density. Yet, solids of different
densities do not separate by density. For example, aluminum is less dense than iron. Wood
is even less dense than iron or aluminum. Yet,
when these substances are placed one on top
of another, they do not separate by densities.
Figure I
Why?
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