13.3 Buoyancy The forces from pressure acting on the bottom of this golf ball are greater than those on the top. This produces a net force— called the buoyant force—that acts upward on the ball.
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13.3 Buoyancy
The forces from pressure
acting on the bottom of this
golf ball are greater than those
on the top.
This produces a net force—
called the buoyant force—that
acts upward on the ball.
13.3 Buoyancy
What is the effect of buoyancy on the
apparent weight of an object?
Buoyancy is the ability of a fluid to exert an
upward force on an object placed in it.
Buoyancy results in the apparent loss of
weight of an object in a fluid.
Buoyant Force
13.3 Buoyancy
Every object in a fluid experiences buoyancy.
• Water pressure increases with depth.
• Forces pushing up on the bottom of the object are
greater than the forces from pressure pushing down
on the top.
• This upward force, which acts in the opposite
direction of gravity, is called the buoyant force.
Buoyant Force
13.3 Buoyancy
According to Archimedes’ principle, the buoyant
force on an object is equal to the weight of the
fluid displaced by the object.
• A submerged object pushes aside, or displaces, a
volume of fluid equal to its own volume.
• A floating object displaces a volume equal to the
volume of the part of the object that is submerged.
Archeimedes’ Principle
13.3 Buoyancy
How can you determine if an object will float
or sink in a fluid?
If an object is less dense than the fluid it is
in, it will float. If the object is more dense
than the fluid it is in, it will sink.
Density and Buoyancy
When the buoyant force is equal to the
weight, an object floats or is suspended.
When the buoyant force is less than the
weight, the object sinks.
13.3 Buoyancy
Two forces act on every object in a fluid—weight
and the buoyant force.
• The force of gravity, equal to the object’s weight,
acts downward on the object.
• The buoyant force, equal to the weight of the volume
of displaced fluid, acts upward on the object.
Density and Buoyancy
13.3 Buoyancy
Suspended
An object that has the same density as the fluid it
is submerged in will be suspended (it will float at
any level) in the fluid.
• The buoyant force acting on the suspended object
exactly equals the object’s weight.
• Submarines and some fish are able to suspend
themselves in water partly by adjusting their density.
Density and Buoyancy
13.3 Buoyancy
Sinking
When a ship’s weight becomes greater than the
buoyant force acting on it, the ship will sink.
As a sinking ship takes on water, the ship
displaces less water, and the buoyant force
decreases.
Density and Buoyancy
13.3 Buoyancy
Floating
A solid piece of steel sinks in water. A heavy steel
ship floats because of the shape of its hull.
The hull is shaped so that it displaces a large
volume of water, creating a large buoyant force.
Density and Buoyancy
13.3 Buoyancy
The weight and buoyant force determine if an
object sinks or floats.
Density and Buoyancy
Buoyant force
Weight
13.3 Buoyancy
The weight and buoyant force determine if an
object sinks or floats.
Density and Buoyancy
Buoyant force Buoyant force
Weight Weight
13.3 Buoyancy
The weight and buoyant force determine if an
object sinks or floats.
Density and Buoyancy
Buoyant force Buoyant force
Buoyant force
Weight Weight
Weight
13.3 Buoyancy
Objects also float more easily in dense fluids.
• The denser the fluid is, the greater is the weight
displaced. The greater displaced weight results in a
greater buoyant force.
• This is why it is easier for a person to float in very
salty water. The dense salty water produces a larger
buoyant force when displaced by the person's body.
Density and Buoyancy
13.3 Buoyancy
The exposed green and red stripes on the ship’s
hull indicate that the ship is riding high in the
water. If the cargo ship were completely loaded, it
would need to displace more water in order to
float.
Density and Buoyancy
13.3 Buoyancy
Assessment Questions
1. According to Archimedes’ principle, the buoyant
force acting on an object is equal to
a. the weight of the object.
b. the pressure exerted by the fluid at the point of contact
with the object.
c. the pressure exerted at the bottom of the fluid.
d. the weight of the fluid displaced by the object.
13.3 Buoyancy
Assessment Questions
1. According to Archimedes’ principle, the buoyant
force acting on an object is equal to
a. the weight of the object.
b. the pressure exerted by the fluid at the point of contact
with the object.
c. the pressure exerted at the bottom of the fluid.
d. the weight of the fluid displaced by the object.
ANS: D
13.3 Buoyancy
Assessment Questions
2. How much fluid is displaced by an object that is floating, partially submerged, in a fluid? a. a volume of fluid equal to the volume of the object
b. a volume of fluid equal to the volume of the submerged part of the object
c. a mass of fluid equal to the mass of the object
d. a mass of fluid equal to the mass of the submerged part of the object
13.3 Buoyancy
Assessment Questions
2. How much fluid is displaced by an object that is floating, partially submerged, in a fluid? a. a volume of fluid equal to the volume of the object
b. a volume of fluid equal to the volume of the submerged part of the object
c. a mass of fluid equal to the mass of the object
d. a mass of fluid equal to the mass of the submerged part of the object
ANS: B
13.3 Buoyancy
Assessment Questions
3. An unknown substance is suspended in water.
What can be concluded about the substance’s
density?
a. The substance’s density is greater than water’s density.
b. The substance’s density is less than water’s density.
c. The substance’s density is equal to water’s density.
d. Nothing can be concluded about the substance’s
density.
13.3 Buoyancy
Assessment Questions
3. An unknown substance is suspended in water.
What can be concluded about the substance’s
density?
a. The substance’s density is greater than water’s density.
b. The substance’s density is less than water’s density.
c. The substance’s density is equal to water’s density.
d. Nothing can be concluded about the substance’s
density.
ANS: C
13.3 Buoyancy
Assessment Questions
4. A 10,000-N ship is floating in the Great Salt Lake,
whose salty water is denser than pure water.
What is the buoyant force acting on the ship?
a. 1000 N
b. 8000 N
c. 10,000 N
d. 12,000 N
13.3 Buoyancy
Assessment Questions
4. A 10,000-N ship is floating in the Great Salt Lake,
whose salty water is denser than pure water.
What is the buoyant force acting on the ship?
a. 1000 N
b. 8000 N
c. 10,000 N
d. 12,000 N
ANS: C
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