CYCLE 5
Developing Ideas
ACTIVITY 1 HW: Density, Sinking and Floating--
KEY
Name:________________________________ Date:_______________ Group: ______
Purpose
You have observed that, when placed in water, some objects float and others
sink. The purpose of this homework activity is to answer the following
question.
How can we predict whether an object will
float or sink?
Initial Ideas
Imagine you immersed an object
in a large tank of water, and then
let go. What kind of information
would you need to know about
the object in order to predict in
advance whether it will rise to the
top and float, stay where it is, or
sink to the bottom? Why do you
think so?
Possible student answers: what material is the object made from, how much the object weighs,
how much air is in the material.
Collecting and Interpreting Evidence
Experiment #1: What determines whether an object sinks or floats?
When you place an object completely under water, it displaces a volume of
water equal to its own volume. Since both the object and displaced water
have the same volume, comparing the mass of the object to the mass of the
© 2007 PSET
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Cycle 5
water displaced is the same as comparing the density of the object to the
density of the surrounding water.
Play the C5Act1HW.mpg movie. In the movie, the person places three baggies
filled with different liquids in a tank of water. The water in the tank has a
density of 1.00 g/mL. The names and densities of the three liquids are listed
below.
In the table, record whether each baggie sinks, floats, or remains where it
was placed.
Liquid in Baggie
Density
What does the baggie actually do?
Plain water
1.00 g/mL
Remains where it is placed
Salt water
1.20 g/mL
Sinks
Rubbing alcohol
0.786 g/mL
Floats
If the density of the object is greater than the density of water, would the
object sink, float, or remain submerged where it is?
If the density of the object is greater than the density of water, the object will sink.
If the density of the object is less than the density of water, would the
object sink, float, or remain submerged where it is?
If the density of the object is less than the density of water, the object will float.
If the density of the object is equal to the density of water, would the
object sink, float, or remain submerged where it is?
If the density of the object is equal to the density of water, the object will remain submerged
where it is.
Although the demonstration was done in a tank of water, the results can be
made more general. Whenever an object is immersed in any fluid (liquid or
gas), to decide whether it will rise up, sink down, or remain where it is, you
need to compare its density to the density of the surrounding fluid.
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Activity 1 Homework
•
•
•
The object will rise up and float if its density is less than the density of the
surrounding fluid;
The object will sink if its density is greater than the density of the
surrounding fluid;
The object will remain where it is if its density equals the density of the
surrounding fluid.
Summarizing Questions
S1. Suppose a block of material floats in liquid A, but sinks in liquid B. What
can you say about the relative densities of the two liquids and the
material out of which the block was made?
Since the material floats in liquid A, it is less dense than liquid A. Since the material sinks in
liquid B, it is more dense than liquid B. We can also write this as: Density Liquid A > Density
Material > Density Liquid B
S2. Hydrogen
gas,
despite
its
high
flammability, was once used to fill airships.
Following the Hindenburg disaster on May
6, 1937, hydrogen was replaced with
nonflammable helium gas.
Compare the densities of air and helium
(C5A1 Table 7). Why do you think helium
is used to fill modern-day airships, such as the Goodyear blimp?
Like hydrogen gas, helium gas is less dense than air and, therefore, will cause the airship to
rise (or float) in air.
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Cycle 5
S3. Ships are commonly made of steel. Compare the densities of water and
steel (C5A1 Table 4).
a) Would you expect a block of steel to sink or float in water? Why?
A solid block of steel will sink in water because its density is greater than the density of
water.
b) Why does a steel ship float? (Hint: think about what else takes up
space in the ship besides steel.)
A steel ship is not a solid block of steel. A steel ship is a steel shell that is filled with a
large volume of air. The air inside the ship is significantly less dense than water. Thus,
the average density of the entire object (the ship) is less than the density of water, so it
floats in water.
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