What is Buoyancy?
 Buoy = float (in Latin)
 Archimedes discovered
this with the golden
crown. (He then ran
around buck nekkid!!!)
What is Buoyancy?
 Buoyancy= “the floating force”
 Water is “heavier” than the
object…so the object floats
 Low density-more likely to float
What are the units for
Buoyancy?
Buoyant force is measured in
Newtons (N)
What is displacement?
 Displace = push away
 The amount of water
displaced by an object tells
you the volume of that
object.
 The weight of the
displaced water is the
buoyant force acting on
that object.
How much fluid was displaced
here?
What is buoyant force?
 Buoyant force is the upward force of a
fluid on an object.
What is a fluid?
A fluid is any liquid or gas.
What are the types of
buoyancy?
 Positive buoyancy:
Buoyant force is greater
than weight so the object
floats.
 Neutral buoyancy:
Buoyant force is equal to
weight so the object is
suspended in the fluid.
 Negative buoyancy:
Buoyant force is less than
weight so the object sinks.
When does an object FLOAT?
An object will float if the
buoyant force is greater
than or equal to the weight
of object.
WEIGHT
BUOYANT FORCE
When will an object SINK?
An object will sink if the
buoyant force of the object
is less than than the weight
of the object.
WEIGHT
BUOYANT FORCE
How do you Calculate BF?
Buoyant Force= Weight of displaced fluid
OR
BF = Wair - Wwater
Buoyant
Force
= Weight of
object in air
-
Weight of object
in water
Buoyant Force =
Weight of Displaced water
•When the object is
submerged in
water, it displaces
an amount of water
that weighs 3 lbs.
6 cm3
6 cm3
•Therefore the
buoyant force
pushing up on the
object is
Buoyant Force =
Weight of object in air minus weight of
object in water
•Wair = 7 lbs
•Wwater = 4 lbs
•Buoyant Force
= 7-4 = 3 lbs
6 cm3
6 cm3
Volume of
object=Volume of
displaced water
Why do things float?
1.
Things float if they are less dense than
the fluid they are in.
2.
Things float if they weigh less than the
buoyant force pushing up on them.
3.
Things float if they are shaped so their
weight is spread out.
Such as these:
Hot air balloons: heat to go up
Submarines:
 fill tanks with air to surface
 fill with water to submerge
Density
Lowest Mass, Lowest Density, Floats
Ballast tanks
Low Mass, Low Density, Neutral
High Mass, High Density, Sink
Sub’s carry compressed air tanks to re-fill their ballast tanks
Density
Air Density= 0.04 g/ml
Balloon
Density =
0.045 g/ml
SINK
Density
Air Density= 0.04 g/ml
Balloon
Density =
0.040 g/ml
NOT
MOVING
UP OR
DOWN
Density
Air Density= 0.04 g/ml
Balloon
Density =
0.035 g/ml
FLOAT
Shape
 How can you get 5000 pounds of solid
steel to float?
Ouch!
You know the
Buoyant Force.
Soooooooooooo……
 A rock has a mass of 76 g
and displaced 80 mL of
water when it was
submerged. What is the
volume of the rock?
 If it displaced a volume of
80mL, then the volume of
the rock is 80mL! Or 80 cm3
And…………
 A volume of 200 mL of water is
displaced by a duck that weighs
10N. The weight of the displaced
water is 20N. What is the
buoyant force on the duck and
will it sink or float?
 BF = 20N
 The duck will float because 20N (weight
of the displaced fluid) is greater than 10N
(weight of the duck).
There’s more……..
 A little 27g fish displaced a volume of water
that weighed 25N. The weight of the fish was
30N. What is the buoyant force on the fish
and will it sink or float?
 BF = 25N
 The fish will sink because the weight of the fish is
greater than the buoyant force.
Some More…….
 A person weighs 250N while
swimming in the dead sea. When
outside of the water they weigh
600N. What is the buoyant force
acting on them? Will they sink or
float?
 BF = Wair – Wwater = 600 – 250 = 350N
 The person will float because their
weight in the water is less than the
buoyant force.
Another?????????
 A dog jumps into a
pool. The water holds
him up because he only
weighs 10N in water.
On land, he weighs
45N. What is the
buoyant force?
 BF = 35N
Observation Time!
Observation Time!
Observation Time!
Observation Time!
Don’t forget your conclusion: