Transcript Standard EPS Shell Presentation

12.3 Buoyancy is a force
Buoyancy is a measure of the
upward force a fluid exerts on an
object that is submerged.
The water in the pool
exerts an upward
force that acts in a
direction opposite to
the boy’s weight.
12.3 Volume and buoyancy
The strength of the buoyant force on an
object in water depends on the volume of
the object that is underwater.
As you keep pushing downward on the ball, the
buoyant force gets stronger and stronger. Which
ball has more volume underwater?
12.3 Weight and buoyancy
In the third century BC, a
Greek mathematician named
Archimedes realized the
relationship between
displaced fluid and the
buoyant force
Archimedes’ principle states :
The buoyant force on an
object is equal to the weight
of the fluid displaced.
So a rock that displaces 9.8 N of water will have a
buoyant force of 9.8 N and weigh 9.8 N less while
under water.
12.3 Weight and buoyancy
Buoyancy
explains why
some objects sink
and others float.
Whether an object
sinks or floats
depends on how
the buoyant force
compares with
the weight.
What will happen if the buoyant force on an
object is equal to its weight?
 It Floats!!
What will happen if the buoyant force on an
object is less than its weight?
 It Sinks!!
What will happen if the buoyant force on an
object is MORE than its weight?
 It will pop up, like a beach ball when pressed
under water. It only needs to displace its
weight!!!
12.3 Density and buoyancy
What can you say about the
average density of these blocks?
12.3 Density and buoyancy
If you know an object’s density you
can quickly predict whether it will
sink or float.
Which ball will sink in water?
Which ball will float in water?
12.3 Density and buoyancy
When they are completely underwater,
both balls have the same buoyant
force because they displace the same
volume of water.
12.3 Density and buoyancy
Average density helps determine
whether objects sink or float.
 An object with an average density
GREATER than the density of water will
sink.
 An object with an average density LESS
than the density of water will float.
12.3 Boats and average density
Use your understanding of average
density to explain how steel boats
can be made to float.
12.3 Boats and average density
If you have seen a loaded cargo ship, you
might have noticed that it sat lower in the
water than an unloaded ship nearby.
This means a full ship must displace more
water (sink deeper) to make the buoyant
force large enough to balance the ship’s
weight.