Properties of Fluids
Buoyancy: Floating and Sinking
SCI 8: Fluids Unit
Curriculum Outcomes Addressed:
- Describe situations in life where the density of substances naturally changes or is intentionally changed (307-10)
- Identify questions to investigate arising from practical problems involving floating, sinking, and density (208-2)
- Describe the movement of objects in terms of balanced and unbalanced forces (309-2)
- Provide examples of technologies that have been developed because of our understanding of density and buoyancy (111-1)
• We will study the following properties of fluids:
1. Density
2. Buoyancy (Floating and Sinking)
3. Displacement (Archimedes’ Principle)
4. Viscosity
5. Pressure (Pascal’s Principle)
Key Terms
• Buoyancy: The upward force on an object
submersed in a liquid
• Gravity: The downward force on an object
Floating and Sinking: Density
• Whether an object floats or sinks is dependent
on the object’s density compared to the density
of the liquid in which it is immersed.
• If the density of the object is greater than the
density of the liquid in which it is immersed, then
the object will sink.
• If the density of the object is less than the
density of the liquid in which it is immersed, then
the object will float.
Floating and Sinking: Buoyancy
• The relationship between the object’s density and the
liquid’s density in which it is immersed affects the force
of buoyancy acting on the object. This is what causes
the object to either float or sink in the given liquid.
• If the object has a greater density than the liquid, then
the gravitational force pulling down on the object is
greater than the buoyant force pushing up, which
causes the object to sink.
• If the object’s density is less than that of the liquid,
then the buoyant force will be greater than the
gravitational force pulling down, causing the object to
float.
The Forces at Work in Buoyancy
• Gravitational force “pulling” down
• Buoyant force “pushing” up
If the buoyant push upward
is greater than the
gravitational pull downward,
then the object will float (if
the object’s density is
greater than the density if
the liquid in which it is)
…and vice versa.
Three Types of Buoyancy
• Negative Buoyancy: Object sinks (Gravitational “down” force is
greater than buoyant “up” force – object’s
density is greater than that of the fluid in which
it is immersed)
• Neutral Buoyancy: Object is suspended in middle (Gravity and
buoyancy are equal – same densities)
• Positive Buoyancy: Object floats (Buoyant “up” force is greater
than the gravitational “down” force – object’s
density is less than that of the fluid in which it is
immersed)
 POSITIVE BUOYANCY
 NEUTRAL BUOYANCY
 NEGATIVE BUOYANCY
Virtual Floating and Sinking Lab
Why do Things Float?
http://glencoe.mcgraw-hill.com/sites/0078741858/student_view0/unit1/chapter3/virtual_labs.html#
Altering Buoyancy in Everyday Life
• Buoyancy can be altered (changed) if necessary.
• Some examples are in which this happens, are:
i. Scuba divers
ii. Submarine
iii. Fish
i. Scuba Divers – Weight Belt and BCD
• Scuba divers wear weight belts so that they are more
dense, allowing them dive downwards more easily.
• Scuba divers also wear a buoyancy compensator (also
known as a “buoyancy control device”, or BCD). The
BCD is a vest/jacket that contains a ‘bladder’ to help
alter buoyancy. The buoyancy is controlled by adjusting
the volume of air in the bladder. When the divers need
to become less dense (to rise), they let air into the
bladder. To become more dense (to sink or remain
neutrally buoyant), they pump air out of the BCD.
BCD 
Weight Belt 
ii. Submarine – Ballast Tank
• Ballasts act as weight and alter buoyancy. Tanks of water
act as ballasts on submarines.
• Changing the amount of water in the ballast tank of a
submarine allows a submarine sink or rise. In order to
sink (become more dense), the ballast tank is filled with
water. In order to rise (become less dense), the water is
pumped back out and is replaced by air.
Practice Questions
1) Object A has a greater density than the liquid in
which it is immersed. Will object A sink or float in
this liquid?
2) Object B’s density is smaller than the density of the
liquid. Will object B sink or float in this liquid?
3) Object C’s density is the same as that of the liquid.
What happens to this object?
Practice Questions
4) When the buoyant force is greater than the
gravitational force, the object ____________.
5) When the gravitational force is greater than the
buoyant force, the object ________________.
6) When the gravitational and buoyant forces are
equal in strength, what happens to the object?
Density & Buoyancy Lab Activity
Boat Building Challenge