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12/8/05
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Page 120
Floating and Sinking
How can you determine if an object in a fluid will float or sink?
You must compare the buoyant force pushing the object upward with
the downward force of gravity, or weight of the object. If the two are
equal, the object will neither rise nor sink. If the weight is greater, the
object will sink. If the buoyant force is greater, the object will rise to
the surface and float. An object floating on the surface of a liquid has
no net force on it.
weight
buoyant
force
no net force
weight
buoyant
force
net force
CALIFORNIA
Focus
The giant octopus, whose
habitat ranges from
California to Asia, is a
relative of the nautilus.
Like the nautilus, it uses jet
propulsion to move around.
A sea animal called a nautilus moves mostly by shooting water out
in a jet through a part of its body that is like a funnel. In order to move
effectively, the nautilus’s weight must balance the buoyant force. The
nautilus moves best when it does not rise or sink. The nautilus adjusts
its buoyancy by changing how much water or gas fills its shell.
1
If the nautilus weighs less than the buoyant force, it tends to float
upward. The nautilus can increase its density by sucking water
into its shell. This water is transferred to the shell chambers by a
tube called the siphuncle.
2
When the nautilus weighs more than the buoyant force, it tends
to sink downward. The nautilus decreases its density by removing
water from the chambers in its shell.
The buoyant force depends on the volume of an object, and the
weight depends on the mass of the object. Recall that density is mass
per unit volume. Whether or not an object will float or sink depends
on the relative densities of the object and the fluid. If the object is
denser than the fluid it is in, it will sink. A glass marble placed in a
beaker of water sinks to the bottom of the beaker. The buoyant force,
which is equal to the weight of the water the marble displaces, is
smaller than the weight of the marble. As a result, there is a net
downward force on the marble. That is why the marble sinks.
check your reading
120 Unit 1: Motion and Forces
How does density determine if an object floats or sinks?
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